You have quite the range of possible upgrades, from APS-C to FullFrame to m43 !
The M5 vs the M5 MkII differences are pretty clear-cut - first page on the DPR review of the MkII highlights these differences.
It really comes down to what you shoot and what you care about.
Going FullFrame means you care about low-light image quality and the shallowest depth-of-field possible (ignoring medium format.) It also means you're OK carrying larger cameras and lenses.
Going m43 means you care about getting great image quality in all sorts of lighting conditions, but falling back a bit in the dimmest of scenarios compared to FF or APS-C. Again, going to DPR and comparing the m43 in either the M5 or M5MkII compared to any recent APS-C and FF bodies will let you easily see the differences.
If you're going to mate the excellently reviewed Oly 12-40 f2.8 on either M5 body, you're going to have a super powerful imaging machine.
I own both a FF, APS-C and m43 body ( D610, D5200 and the Oly EM10 ) and split my time between them - probably 25% FF and the rest between the others.
I could easily live with any of them. There isn't bad cameras nowadays... just cameras that don't fit your hands.
🙂