I would use a 3 roller on anything thin wall. My 3/8" was 0.049 wall. (Hence my 150' IC)
Sweet roller by the way - that makes that 1/2" thin wall tubing more attractive.
Flap disk or circle disk in the 30-60 grit range will be the fastest way. Consumable will be the cheapest also..
Even with a plasma, you are going to need a guide ring. For that small of an opening I would still reach for the grinder. Mark your new hole, and go at it.
I did the same as franksnbeans for my IC. Its tough. Take your time.
You could make it easier by finding a post sticking out of the ground and walking the tubing around, keeping tension. (Like at a gas station, or car wash.. About 3 feet out of the ground, and very solid.)
Its ugly, but it...
I use a porta-band. I got rid of my abrasive chop saw a long time ago..
For the stainless I use the plasma..
It also depends on how you plan to weld.. How close your joint needs to be..
3/8" Copper tubing and single flare fittings on mine.
Build list of fittings in my thread.
https://www.homebrewtalk.com/f51/my-first-single-tier-297314/