You know, it's starting to be hard to find good suggestions to add. A lot of current complaints seems like things that are planned to be fixed in the future, but the features that do so aren't in the game yet. Like; the mine and farm 10% attrition rate is too high, and considering villagers are the only source of non-valuable slaves you can send in. Which makes fully utilizing even a lv.4 farm impractical. But that's going to be fixed with the alchemy and forge upgrades. Slave production and colonization is another issue, since getting the x+ slaves you need for a given colonization option is... difficult. But that whole system is going to get changed, plus the addition of new countries adding new possible sources for slaves. Need more knights? Surely another country is capable of producing them, so just invade that country.
As such, here's probably my last observation/suggestion before the game gets updated; you can reasonably expect to beat mondstad's 3 initial stages by around week 40, since you just need a couple defensive Mitachurls to do so. But conquering the final stage effectively requires a strong alpha-strike (when not on easy), I.E. Lawachurls. And for those? You can potentially see them sometime around week 80, but I'm going to say that 90-100 is a bit of a more reasonable estimate. And while that's not really a problem... it does mean that you'll need to slog through 50-60 tedious battles until you are actually in a position to take the city. And fighting the same enemies, with the same units, get's really tedious.
So, here's a suggestion; once a city becomes "locked off", reduce the spawning of new revenge armies to every other week. This would reduce the number of battles the player would need to play through, while also giving the threat meter more time to build up. So you'd see more powerful teams attacking overall, instead of dealing with mostly knights and elite adventurers. It'd make the defense less tedius, while also potentially reducing the amount of actions needed to replace attrition. Which could help prevent the player from feeling "bogged down" while they wait on their breeding program to actually produce the units they need to finally finish conquering the country.
Alternatively; it might be worth looking into how series like Total War do their auto resolve battles. Currently auto resolve is just a pure numbers game. Power vs. Power. But that results in both sides taking more losses than could reasonably be expected. It's why I don't use it when defending chockpoints - I've typically got a good enough defense that I'd expect to take minimal losses, while the auto-resolve is likely going to lose me one or more units. An improved auto resolve would help to reduce the tedium of fighting the same battles multiple times.