Tobin a competitive setting demoman is probably the hard-to-master class with a high skill ceilling you're looking for.
The competitive scene also needs more demomen so if you want to get into comp in a couple of months once you're getting the hang of it you'll be able to find a team relatively easy
From the PUGs I've done and watched, getting a Medic always seems harder than finding a Demoman. Regardless, they are in really short supply so you can get away with being a meh Medic (Like me!) and still being acceptable.