For me, this is not my first game which develop from Ren'py engine that I have been play. I see your point about users issued problems in some platforms (especially on Android) but I think it hard to transfer this game to other engine because a different engine use a different language (I mean like Ren'py use Python). And come to think of it, I don't see the way to transfer save files we have been play from Ren'py to another engine.
And about big and complicated problem, I think Ren'py can afford it. I have seen many games which develop from Ren'py and being great. For example, Doki Doki Literature Club was developed from this engine too.
And last, I agree with you about supporting the developer. This game is deserve to get support for more CGs/OST. (Of cause, if you can.)