1. The P3 camera is generally considered to be fair to poor, especially compared to modern cell phone cameras or, gasp, real DSLRs (you want to fly a DSLR, you want an Inspire or better).
2. You can take pretty good web sized pics on a P3. Blowing them up for printing or pixel peeping, not so much.
3. We have seen a number of P3s with misaligned lens elements (on side sharper than another). That potentially can be fixed by taking the camera apart and reshimming or, more practically, sending it back to DJI. Who will send you another camera / gimbal and you have crap shoot as to whether or not it's better.
4. There is a generic DJI lens correction file for Adobe Camera raw. It sort of works but mostly seems to fix some center of lens mustache distortion that is only apparent if you are shooting gigantic straight lines. One of my many 'projects' is to go back and make a set of correction files to see if can be improved. But that's an entire day project.
5. As has been mentioned, DON'T use ND filters on stills unless you really understand what you are doing (actually trying to blur an image). NDs are useful in video to keep the shutter speed DOWN so the video looks 'film like" (look it up). In general, you want the fastest shutter speed you can get away with to minimize vibration. Polarizers are a bit more complex. They can help over water but, again as has been noted, they only work well in one direction. So if you yaw too much, you just change the exposure which can be annoying in post. I've used them on occasion. It can help. It usually doesn't. Try it.
6. Shoot in DNG ('raw') and post process. At the very, very least turn sharpness off and add it back in post since sharpness settings depend critically on how big the final image is and where it will be displayed.
7. The Automatic Exposure Bracketing (AEB) doesn't work very well at all. Ignore it. Just work on getting the exposure correct. Turn the histogram on....
8. It's a better video camera than a still camera. Much like a GoPro. Deal with it.
9. If you are worried that something has changed / isn't right, take some pictures of a brick wall with the P3 on a sturdy platform to rule out a vibration issue.
10. There is also some reasonably good evidence on this forum that the P3 camera does much worse in the cold. Possibly due to some differential expansion / contraction of the very light, flimsy bits on the camera. Weight reduction hath it's disadvantages. But it's summer now (at least in the Northern Hemisphere).