+1, before going from obsolete (meaning not supported with good selection of quality parts) system to another soon to be one, skip 8s and go straight to 9s. The wheel is the same, so it's only the shifter, and cassette, and the price for comparable level is going to be about the same. Smart move to stay with fat tires.

The only other suggestion, id keep in mind what other work the bike may need now or in the near future, and compare the total estimated cost to what a new bike might run. It can often be expensive to keep old stuff alive and you can end up being able to ride new for about the same price as riding used. Not saying that's your situation now, but keep it in mind.