Verified that ECC is working with the 5750G (PFEH OFF) by overclocking the system memory from DDR4-3200 to DDR4-3600 while staying at the default DDR4 memory voltages (1.2 V).
No crashes or non-correctable errors, MemTest86 Pro correctly noticed ECC intervening (hypothesis: Regular operating systems should also be able to properly log ECC errors):
Will look at the 5950X next.