What is the best strategy for this layout if I want/need to remove copper plumbing and move to PEX? (two bathrooms connected by the same supply pipes)
That kind of corrosion is usually attributed to the failure to remove excess soldering flux, and it does not indicate the pipes are corroding through. Wiping all those areas with a wet rough rag would probably remove the deposits. Then examine the joints for pitting and for pinhole leaks. At this point replacing all this plumbing would be premature.
Copper water piping can corrode through. We had multiple pinhole leaks in the 6 ft vertical run carrying hot water from the tank down to the slab. But your pipes do not appear to be corroding through. The corrosion is probably superficial and stopped when all the excess flux was consumed. The excess flux should have been wiped off by the plumber who sweated the joints,  but often this is not done.