First off, it’s a shame that it’s all gone this way and I understand that a malfunctioning and underperforming device can be frustrating - so please take the following as replies rather than rebuttals. I’m hoping they may be of some use to you, or to other frustrated FP2 users reading this.
It’s a bad idea to ignore problems, especially if they pop within months of buying the phone. This may have been a factory fault and allowing it to persist may have lead to larger issues. That two year warranty is included in the price of an FP2 and you should never hesitate to use it.
Again, this sounds like there was something wrong with your device. It’s a shame you’re out of warranty now because it’s likely you could have had it repaired at no charge had you taken action at the time.
Try screwing in the bottom module more loosely. That’s done the trick for me on a couple of ‘broken’ microphones. Academic, I know, as you’ve already started using a different device, but still worth a tinker.
Coupled with the random reboots you mentioned before, maybe try a factory reset? That usually gets rid of random reboots. You may also want to try using a shim to secure the battery better.
But apart from that: yes, it is an aging phone that cannot run all the things a brand new flagship can. So some clever husbandry when it comes to installing apps is required - which may disqualify the FP2 for some.
Unless you replaced the old module with another old module, the 12MP module definitely does improve on the 8MP module. Here’s a gallery of FP2 images for reference. Are you unable to get good results with the camera?