Kernel panic, even after clean install on 2015 iMac

SOLVED!! (kind of)

If anyone comes across this post and has the same issue, this is what I did to stop all the crashing and kernel panics:

I clean installed macOS High Sierra 10.13.6 (by wiping and installing via the internet in recovery mode) and now both the iMacs that used to have massive issues haven't crashed since reverting to High Sierra.

When I wiped the computers I also made sure the HDD's were re-formatted to HFS+ as they were converted to APFS when they upgraded to Mojave. Not sure if APFS was the issue or maybe the graphics cards, but four other iMacs that have more solid state storage run fine on Mojave, including an older 2014 iMac. Now the 1TB Fusion Drive and 1TB HDD are HFS+ and no crashes or kernel panics.

Thanks, I have previously thought it may be the RAM, so tested with the Apple diagnostics and showed up fine, but I guess that doesn't mean a lot? It is OWC RAM, and it could be the issue, but I've been trying to replicate the crashing and only seems to happen when we are legit trying to do work and not when I'm trying to force a crash of course, so very hard to test! (And this is why I took so long to reply as I haven't had the time to constantly swap modules and wait for a crash)

To provide some more background we have 5 iMacs and all have factory and OWC RAM and the two 2015 iMacs have the same issue of restarting randomly during renders, and the 2017 iMacs do not, so it could be a coincidence and they may just have bad RAM, but I was thinking it might be some issue with the AMD Radeon R9 M390 2GB as the 2017 iMacs have newer video cards.

If anyone knows an easier way to test RAM than just swapping out modules and waiting for a crash then please let me know :)

After reverting back to High Sierra and converting the HDDs back to HFS+ the issue has been solved, so could have had something to do with the HDDs... Two iMacs out of six had this issue and they are the only ones with out SSDs so makes sense it would be the HDDs.