Other than tires, other "upgrades" will have a low return vs $ invested. (Kool Stop brake pads IF you feel your braking is too weak)
I think a more useful approach would be to invest in decent tools.
First, tools to service the hubs. Cone wrenches, Free Wheel remover, grease & bearing balls.
Most factory hubs tend to have minimal grease and that would be a good place to start.