Some thoughts after playing for a bit:
That's very solid feedback, thanks. It took me longer to read this as I was busy working on version 0.39.
A smartphone Health App that depicts all of Oniichan's stats is already on the agenda. It will list things like your achievements and maximum energy.
The Lewd Conversation follows the same rules as the Casual Conversation but with higher failure rates. This will be addressed in 0.39 with lower failure rates and more closely tied to her Trust. Very higher Trust will open her up to Lewd topics at the same rates as Casual topics. This is a placeholder fix that is serviceable to unlock all lewd content. In 0.4x, the Lewd conversation mechanic will be revamped to give you more agency to do the corrupting instead of relying only on her interests.
There will be a maximum negative Attraction in 0.39, capped at either -20 or 0, depending on your previous relationship choice in the New Game setup.
The RNG for Casual Conversation is skewed in favour of winning, and as you level up your Trust, good results become more likely and bad results less likely. If you had ~120 Trust you'd still be fairly low level for 0.38: 1, 100, 250, and 500. These values are tweaked in 0.39 to 1, 80, 200, and 400. The results of positive and negative outcomes are also being made more predictable from -4 or -2 in with low trust to -2 or -1 with high trust.
Her Mood influences everything, and in 0.38SS, it is already the case that a good mood protects against bad conversation results.
New mechanics to gain Attraction are introduced in 0.39 as well, like skinship.
I do want to work on the QoL of the Banking App, but that's not super high on my priorities list, currently.
Closing off: the strategy should be pretty clear now. Increase her mood to normal or superb, and then gain her trust through interactions. If an interaction goes bad, it lowers her mood instead.
Great feedback overall and much overlap with what I had in mind already, which is encouraging. I hope you'll stick around for the development ride as I flesh out the game further. Undoubtedly, more feedback will be needed in the future!