How about a mixed approach? Maybe something like this: you finish writing for one route (so the story is somewhat linear going forward but at least that one's done) and then work on the other 2 afterwards and coterminously? Then when those are all done you can decide if you want to go back and add more lines to fill them in some, add whatever cutting room floor scenes were skipped, and so on. That way it goes something like 1 year, 2nd year, and then if you decide you're sick of working on it at least you can call it done for your vision, but if you still treat it like a passion project you've left yourself some room to keep at it.
I guess the downside there is you'll have to leave yourself a lot of room in the writing to add whatever was skipped and still have it make sense in the meantime and not reference stuff we wouldn't know about.
Either way the game is amazing and I hope you find a solution you're happy with. I can't speak for everyone else but I'm willing to wait/stand with the game for whatever the duration is. I agree with everyone else that you shouldn't burn yourself out as the most important thing really.