Is plaster repair a DIY job?
I typically use confill ( concrete fill which is a setting compound with fiberglass fibers in it ) to repair plaster as my large gap and then put a sandable top coat like a 30 minute setting compound.
How hard is it to skim coat a wall?  You could learn.  YouTube is amazing.  Start with a small wall.  You'll have to worry about putting extensions on electrical outlets, dealing with trim elements ( do you accept them more recessed with your skim coat build out above ).  Tile Coach has some good videos on tile floating to get walls perfect before tile install.
You should be able to go overtop of paint with a setting compound some people add a wood/glue mixture instead of water when mixing to get better adhesion.
It depends a lot on how wavy the walls are, how close to perfect you want them and how much time you have to spend learning and fixing.