I've read that it's possible to use an external 24V transformer, but
Honyewell tech support just told me in a chat that that's possible only
for 2-wire heat-only systems.; is that true? There are posts and YouTube
videos that seem to suggest otherwise. This is a 4-wire gas and A/C system.

Advice?

Perce

You may be able to connect a new external 24V transformer to the new thermostat terminals R and C. Connect the existing R wire to the R terminal as well.

The thermostat will draw power for itself from the R and C connections to the new transformer.

I give no guarantees that this is right, find some other confirmation.

The phasing of the new transformer relative to the old one __may__ be very important. Connecting wrong may do damage.

This gets complicated, but should be possible.

m