1998 Cadillac Deville 1998 Cadillac Deville Disappearing Co

The car apparently is eating coolant as I need to top off every 20 miles or so. Virtually no evidence on ground of leaking, same for any evidence on hoses, overfill container, engine components etc. Of drips, leakage, etc. Mechanic pressure tested system and found no leaks. Interior climate control system working fine -- running heater high with windows open DOES drop the temp when it gets high (>220). After replacing water pump and thermostat, leading candidate is blown/compromised headgasket, but:

No white smoke out the tailpipe.
Car starts and runs fine
Oil looks fine (not milky / no contamination)

After more or less overheating twice, I now keep the coolant temperature on the Digital Dash at all times and watch this closely.

I have not attempted over the counter sealant but will do that next.

I've been advised against attempting head gasket replacement as it often leads to other expensive findings and car is not worth much more than the $1500 gasket bill. I'm unable to do this repair myself, so I'm beginning to think it's the end of the road for this car, although I'd rather keep it if I can. However, my daily commute is 20 miles and being left on the side of the road twice in the last 2 months is a strong case to move on. Plus constant monitoring of engine temperature while driving is actually quite stressful.

So, where's the fluid going? Any advice? Anything else I should check?