um, the vehicle trick doesn’t work underground, obviously, and sleeping & crafting underground is superior to anything else because NO interruptions!
I suggested a while ago that we add a way to add/remove vehicle batteries without needing extra supplies; that way you could just have the solar recharge station on the surface and your crafting setup underground, and just swap batteries when needed. Not sure if it got implemented or not.
hate to say it, but as long as being underground eliminates all threats from monsters, I’m not really in favor of making it more workable to live underground indefinitely, there should at least be some monsters accumulated on the surface to deal with when you go out for food.
Note I’m not saying it should be impossible to build an impregnable fortress, just that it should require you to build it rather than just walk down some stairs 
also, a muffler shouldn’t decrease smoke output, just noise.
Being underground shouldn’t eliminate all threat of monsters, and wouldn’t with a properly developed z-level system, since being underground wouldn’t cause all activity above to cease. But there should certainly be a way to wire an above-ground solar generator into a below-ground bunker.