Replacing hydronic system air vent valves

Hi everyone — my heating system seems to have two air valves near the boiler, a tiny one sticking out from the boiler itself and a Taco 4900 attached to the expansion tank. I think both are stuck and need to be replaced, the pipes gurgle like crazy whenever the heat is on so I assume there's a ton of air in the system. I loosened the little screws on the valves and a ton of air hissed out, followed by constantly leaking water until I tightened 'em back up. Our water is pretty hard and I've had to replace almost all the faucet cartridges since moving in, so it wouldn't be surprising.

ChatGPT (who is sometimes right and sometimes psychotic) says I should replace the head of the Taco valve (with this I guess?) and either replace the little valve entirely (this or this I suppose) or just cap that pipe.

Does that make sense to the real experts here? Can I just shut off all the ball valves in sight, drain the pipes from the valve below the circulator, unscrew & replace those parts, then open the ball valves and refill to the same PSI (approx ~12 psi/30 ft)?

Anything else I should consider before it starts getting cold out? Should I try to flush air out of each zone manually as well, or just let the new air vents do their job? (I don't have bleed valves at the baseboard radiators themselves.)

Thank you in advance!