I would first just fly for a while with the medium preset and see if that solves the crashing. You can always turn things back up but start there as it might be something else causing crashes and you waste time trying settings with no progress.
If medium is stable then just turn up things one by one. I think ultra anything is asking a bit much of your system honestly. The difference between high and ultra for most things is so tiny it’s not worth the extra stress on the system IMO. Clouds is the biggest difference at ultra but also the heaviest push on your GPU and a bit on CPU.
Good luck with Medium!