My Shimano press-fit BB is running about as smoothly as the trains during a light snow shower. Am I right in thinking that instead of removing the old BB and getting a whole new one pressed in, I can simply order a pair of 6805 bearings and swap them in myself?

IME the cups will disintegrate as you try to get the bearings out. And it’s a PITA to do as there’s barely any of the bearing showing to get a drift onto.

I’d suggest it’s time for an upgrade…

Ok thanks. It looks like the way to go is to order a KCNC unit and then get a set of MRA 2437 LLB from Enduro in the US when the original bearings give up.