Honestly, if you are doing 20 mile commutes, that bike isn't bad as is, aside from replacing/cleaning any old busted parts. Just because you haven't "shifted a gear" or "used the front brake" doesn't mean you never will want to. Maybe you just aren't experienced enough to use them yet, or see the "benefits" of using them. Knowing how things work, and being able to use them, out of "muscle memory" when needed, will make those things seem less cumbersome or unnecessary. As far as paint goes, take it apart, spray paint it the color you want(plan on never being able to sell it) and be done with it. If you feel like you only use/want one brake, use the front brake. Switch it to the side the rear brake is on. Maybe you only use the rear brake because you favor that side for some reason when you are riding, ya know?

If you simply want a single speed bike, get another frame and build one, or just buy one that's already a fg or ss.