Yes, you do it well for now, but once things get more intimate, one would expect conflicts of interest.
No worries and level 10 is the max for now, yes.
I understand branching is time consuming, but it would be worth it in the long run in my opinion. It would also add replayability and make the player more invested in the MC. I don't think there needs to be a story path with every girl in every playthrough. Sure, choosing one would make put the others in minor roles, but it would make more sense from the story perspective. Like you said you have interactions with others at solo girl events, so they can still be important enough.
Well, I suppose I do have a choice in the matter by simply not going after every girl at some point. I would like the drama the consequences brings, but it's not a gamebreaker as nothing is forced, thankfully. I'll just have one MC focused on Luna, one on Gracie, and so on. What eyes don't see, the heart doesn't grieve over.