lol. first off there's nothing wrong with the default naninovel save UI, they've done what they can with how they've made the system. I might touch it up in the future but it's not a priority right now, it fits with the other UI enough for it to not bother me.
Secondly, both the 'back option' and 'feature to hide the dialogue box' complaints that you had, 'shortcomings' you said... both of those are already in the game, hiding the UI is a three finger tap on android and going back in dialogue is a two finger swipe to the right (or left i can never remember).
I'm working on adding a controls screen that shows up for android users exclusively but the game is mainly made with PC in mind so UI can be small at times, though I haven't had any problems with the buttons at the bottom during testing, but maybe it's worth me putting a bit more space between them, I'll keep it in mind.