Good points, but some of them are already covered, so I will answer them :
2. Display Shield HP Value
Show how much HP a shield has and how much damage a unit is dealing when hitting the shield. The barrier thickness is a nice visual indicator, but having actual numbers on the UI would be great.
This is a perk you unlock later in the game.
6. Add indicators to show which girls are not capturable when fighting in combat
This might be due to my complete lack of Genshin knowledge, but some enemies appear like they should be capturable in combat but aren't. Adding indicators to the special girls who can't be captured (in combat) would be nice. e.g. Klee, Ellin, Ematol
Loli characters are not capturable. Special characters are either vision holders (playable girls) or NPCs (unlocks training in second room)
7. Add an instructions page to the home screen
Seems to be a common complaint, but adding a few pages of instructions to the home screen would be very helpful. In game tutorials are better, but far more work. Providing a written guide of the base systems would make the game far more accessible without a ton of overhead. (I hate writing documentation too, but it is real useful)
I did mention something like that, but it was a matter of putting more in evidence the unofficial guide that is already mentioned in the opening post (same post than the download links, just below them). It has everything you want and a lot more. It's just that A LOT of people seem to miss it, so I suggested putting a mention in the menu so that there is no excuse lest for not understanding the games mecanics.
10. Add warning indicator for Klee
She is extremely easy to reach early game and killing her will often result in a game over for many players. It's not obvious that killing her was the reason why either.
Le Readme and the guide both warn you. Lore warns you. If you were not warned enough, I think death is a feature.
11. Prevent Enemy units from bypassing controlled territory
Enemy units shouldn't be able to run past your units and all over your bases. It's annoying to have to spend AP to fight them or send units to chase them down. I suspect this would also be something that could benefit the game systems in the long run by allowing proper establishment of territory and frontlines to strengthen the strategic aspect. It also plays nicely into the concept of abyssalization.
Understandable, but not all units can do it : only wandering type units can bypass the frontline (see enemy teams from the Readme). They act more like enemy scouts that disturbs your territory, so it makes sens they can bypass the frontline.
12. Modify Collei's Behaviour
Having earlier access to a proper tank unit in the form of Dendrochurls would help smooth out the early game fights, compared to where you're stuck with only basic units and ranged fire/ice/electric units.
The issue is that she both takes a long time to reach the player and sometimes she refuses to attack, staying in the Ice region the player can't reach due to the cryo 30 requirement. Having her reach the player earlier and forcing her to attack towards the gorge would resolve these issues.
For early game, the first tier of elemental units is enough as long as you take your time. If Collei does not attack, you have to bait her in doing so.
15. Rebalance Automatic combat resolution
The results between fighting the battle with the auto resolution and starting the battle and running it fully automatic from inside is wildly different. I'd guess this is because auto resolution is using power stats of units which is not a good indicator of how they actually perform.
For me, I'd probably just simulate the auto resolution outcome using the same logic as the current auto battler in the background to give much more reasonable results. Nowadays PCs are more than powerful enough to handle this without the need for a heuristic. Only thing is to make sure to add a fail safe (e.g. to set a max turn count) and then fallback to unit power to prevent infinite loops.
You may be happy to know that an improvement of the auto-resolve feature has already been announced for the next update !
16. Modify Shield Logic
One major hurdle players experience is that enemies can re-apply shields faster than you can break them. This results in battles where a single enemy unit defeats your entire army (e.g. early elite adventures).
This leads to game over scenarios because your threat exceeded the limit a single time. (See Noelle)
What I would like to see is units receiving a debuff each time they are shielded, resulting in the next shield being weaker by a percent. You'd have to mess around to see if a flat debuff (e.g. 10% each time) or some other scaling like 3%, 6%, 9% (3%, 9%, 15% total debuff) or would work better. This could also scale with difficulty.
My main rationale behind this is that it would give large armies a chance to whittle down powerful units at the cost of a large portion of their army. This would give the player a fighting chance against some of the more powerful early enemies.
17. Remove the ability to heal from most enemy units in battle
Same idea as above, removing the ability for most enemy units to heal (some special units can be exceptions) would allow players to beat strong enemies over multiple battles as damage would "stick".
Between 16 and 17 this allows you room to individually buff units or send bigger attack waves and could create a more of a tug of way style gameplay where the player might lose a few battles and then need to reclaim territory as they weaken the enemy units. This would require a bunch of rebalancing in terms of attack wave timing and strength and I could definitely see it frustrating people, but I think it presents much more dynamic and strategic gameplay than stacking all of your powerful units outside Mondstadt. It could also help prevent players from idling and stacking up units infinitely, something the food mechanic was intended to address but has been mostly gimped by the ability to sacrifice.
This is more a matter of taste. Usually, by the time agressive shielded units start appearing, you either have enough to stop them if you go all-out, or you rushed too fast and dug your own grave. For Noelle, yes, she is a monster and probably game overed most players at least once, but she is a good warning to be careful.
Also, I don't think units fully heal after each battle (at least not big ones). I know vision holder take a few turn to recover when they escape a fight.
Lastly, mi-to-late game, shield and healing is what keeps the enemy alive against you.
18. Change Threat Calculation
The current threat mechanic incentives players to sit around and do nothing. Doing so prevents powerful units from spawning, allows the player to increase their element power, stockpile resources and build up an army of powerful units. I generally think encouraging passive gameplay is both boring and bad game design. As I mentioned earlier, the food mechanic was seems to have been intended to prevent players from doing this, but it isn't really doing its job.
Additionally, having the threat jump up and down based on attack waves and certain player actions makes for a rather janky experience where everything is fine one turn, then you hit the next breakpoint and have them mop the floor with you. Then it later falls back into a lower level where the enemies can no longer damage you.
Instead I think calculating threat based off of occupied territory within a nation would make for better gameplay. Each captured settlement would contribute a per turn threat increase. Losing territory could result in a reduction of threat.
This would create a semi self-balancing mechanism where if the player controls a lot, stronger, more aggressive enemies appear but if the start losing a lot the enemies become weaker again, allowing the player to recover. Some logic would need to be added to have powerful units stop attacking / return if the threat drops low enough though.
This also creates room for threat reduction (or even increasing) buildings to be built by the player to help manage this as the game goes on.
As a bonus: This is fairly realistic in narrative. Powerful adventures go out to battle great threats and then leave once it's defeated. After all, they're not going to stay around to farm a bunch of level 1 mobs once it's not a threat anymore.
What you propose is pretty much what already happens.
And while I would enjoy a very agressive enemy, I understand that too much pressure on the player is not everyone's taste.
19. Threat level Power Spikes
As I spoke on above, the threat breakpoints results in massive power spikes when certainly breakpoints are hit and a massive difficulty reduction when you fall below them. It would help smooth out the difficulty curve if there were smaller levels between each existing breakpoint. I think this could be fairly easily done by spawning the new unit type alone, without the supporting army at first. This means a knight spawns without supporting knights, an elite adventure spawns without supporting adventures/elite adventures at first. Then the number of supporting units can increase as threat increases, potentially past the current limit.
I think this is a key part of the design : you know where the breaking points are, and try to avoid them as long as you can. I like you idea of changing the number of supporter in a more continuous way, and I think it could also serve as a good way to ramp up enemy strength late game.
I seem to rebute a lot of things, but keep in mind that what I did not mention, I usually agree with. I hope I don't come out as agressive. It is not my intent.
I put these coments because there are balancing points that change with late game, and I think the unofficial guide serves as a very good tutorial (and a quick glance can save you from a lot of troubles).