Ok, while searching for how to do it on an LS I guess I was only thinking of an old style power flush where you connect a garden hose and plastic piece that went into the radiator fill to aim the overflow away. I didn't realize that's not the only way.
So I guess I could just get a flush agent, run it through the system per it's instructions, then drain and fill the system with water a few times to clean the flush out and then fill with new coolant?
Which flush agent should I get? I don't see any particulates or other trash in the coolant in the degas or fill tube though I never paid attention to it before all this and don't know how it looked if that mechanic drained anything or when I drained it the first time. I think if it had any of that sealer stuff in the system it was probably before I got the car since it has always had a bit of an issue. It would get too hot if I beat on the car a lot from when I got the car. I'm thinking maybe the radiator's been restricted/clogged since I've had it.