FWIW good DDR3200 should be fine (as fine as it is for the 10850k certainly) for Ryzen 5000 and there is a good chance if your cooler has an AM4 mounting kit. I bought one for a realy old Noctua. It was like $5 online.Thanks...it's something to consider.
One thing I think I should make clear, part of the appeal to me for 10850k was to upgrade as little as possible and reuse as much as possible until I was ready to go all in. I can reuse my cooler and ram with a 10850k and I don't know if that would be the case for switching to AMD/5900x. $600 to upgrade to a modern 10core out the door vs $800+ for 5900X.