Those are interesting ideas. I don't think changing from levels to pure XP would really matter much, but perhaps there's something there I'm not seeing right now. It did however make me think about a hypothetical scenario where the player is shown all the options implemented in the game, but with the ones costing more XP than they have being locked or greyed out. This way we could have a better understanding of what we're working towards unlocking, thus helping make the choice.
The extra sections for extended training would be beneficial as well, and it does seem to make sense placing them in periods of longer resting/peace. Like, if the character would have several days ahead without anything relevant happening, kind of time-skip-like.
And regarding the last, if it's going to end with a single ejaculations, maybe it would work better with a pleasure meter system, rather than "choose to have sex > instant orgasm". Like, each option could increase in varying amounts how much pleasure both MC and the partner would get, and when it reaches maximum they achieve orgasm. This would be harder to implement, but I've seen it in a lot of other "training" games and works wonders. For the sake of realism it'd be better if the female can reach orgasm more times than a male, which would also work towards making sure that foreplay doesn't end things too soon, but I realize that this could conflict with choosing a female MC... Or maybe not, since the succubus is draining their energy after all, so it could make sense they get tired faster. But I'm digressing now.
Im glad my feedback was of use, and I'm excited to see the future of this game. Casual Desires as well, but it's a more casual (hehe) experience and I really enjoy these more serious, fantasy stories. Godspeed, ionDivvy