I'm fairly new to zwift, and enjoying it, but the old bike I coopted for it is quickly dying--derailleur not working well or consistently, I think there may be a broken bearing in the bottom bracket bc it's noisy AF, etc. So I would like to replace it, but not put a lot of money into it. I mountain bike pretty regularly, and i don't do any road riding. I sometimes get lower back pain if I spend too much time in a road position, so i probably want a more relaxed geometry. What would you suggest for a relaxed geometry bike that will be cheap but will last a long time as a trainer?

For reference, I use a kickr snap that I take on the road with me pretty regularly.