Before you buy a new phone, try if you can solve the issues you have with your FP2.
Have you tried reinstalling FPOS manually? While this deletes all your data, it also might help solving e.g. reboots.
For battery life, make sure that you avoid these apps if you can, since they need a lot of power. You could also try to install LineageOS which doesn’t include G00gle Services which could also improve battery life. Also, you only need to change your battery every 2-3 years when it starts losing capacity, changing it more often really is just increasing electronic waste.
Regarding the bottom module… Yeah, they are poorly made, I’m also using the fifth one at the moment I think.
I hope you can keep your FP2 for another year or two, if you have any more questions just ask 