It seems to me that there are a couple of things not being considered.

If you are concerned about lack of oxygen in the event of a discharge of a
fire extinguishing agent into the cockpit, then maybe you should consider
the installation of an oxygen system that would allow you to select 100% O2.
This would allow a high concentration of extinguishing agent in the cockpit
without becoming incapacitated.

Second, if the fire is still present after your best efforts, wearing a
parachute (all of the time) would be the option of last resort.

My vote would be: halon, use of 100% O2, egress from the burning airplane
via parachute.