What is the idea behind the hole in the thermostat and why is it not standard on all new thermostats? It certainly can’t allow that much more coolant to flow, I can see it being used as a way to prevent a blockage due to air in the system (which should be able to be bled normally).
I have heard of the DIYer doing the “hole in the thermostat” thing for 30+ years but no professional mechanics (at least that I knew) ever used it.
OP, myself I think you have a headgasket sealing problem that allows exhaust gases to contact the coolant, even in the smallest amount, or a cracked or not flat head.