Bottom bracket upgrade
Forget Campagnolo if you are on a budget. SRAM and Shimano both still make cranks/bottom brackets that are compatible with threaded bottom bracket shells, as do other component manufacturers like FSA.

Presumably you have a square taper bottom bracket axle. You can get a cranks that fit that, but you can run into chainline problems because different cranks require a different axle length. Replacing both the crank and bracket at the same time often makes more sense.

I'd recommend looking at the whole drivetrain an asking yourself if you want to upgrade it as a whole, to get more gear ratios perhaps. That may require replacing many components at once. Just swapping a single component such as the crank may not bring any benefit.