Seems like I managed to resolve the RAM issue reseating them.
I’m still tweaking a few bios settings to get optimal performance out of the system - and realised my fans were running on DC instead of PWN so were much louder than desired.
Plan at the moment is to swap out some of the Corsair fans for quieter Noctua fans, and add the full number the case can support.
Will likely want to add more storage at some point as well - got quite a few M.2 slots available for use.
I ain't quite out of the woods with errors yet - but it's been a fun experience.
The most concerning one was when the motherboard was trying to read a boot device that wasn't there (afaik), so the only thing that loaded was a blank screen and _ cursor. That one persisted for a couple of restarts, and then I figured both the PC and me could do with some sleep - tried again the next day and everything worked.
Fanspeed is about half the RPM it was before - although the system is still louder than I'd ideally like.
I'll create some new threads to deal with the Sleep / Startup problems I'm occasionally getting.