I have an old YHM suppressor mount that I want to remove from this rifle. It wasn’t installed with any thread locker or adhesive, just torque, and it only has 100 rounds or so through it since it was installed. However, I can’t get it to budge just by clamping the barrel. Any advice on how I can get this clunky thing off?

Heat the heck out of it, it’ll bust loose.  You can fab up a V block for the vice from a 2x4.

A barrel vise from wheeler or viper would protect the barrel from slipping and trashing the finish.  Use rosin or sugar with wood blocks to provide a little more bite.  Holding barrel so it won't turn and a sharp whack on a quality, good fitting wrench will loosen it.  If it's been loctited, use a heat gun.