Coolant Issue - 1997 Subaru Legacy Outback 2.5

I've got a 1997 Subaru Legacy Outback (2.5L) with some cooling issues. I've been trying everything I can think of short of a full head gasket swap, which was done about 10k miles ago (along with timing belt, water pump, other disposables). Honestly, If it's truly the HG I'm not going to attempt it again. The flat 4 engines are a huge pain, and given how old the car is I'd probably just part it out or get myself a tax credit.

That said, if anyone has any ideas or things I can try, I'm all ears.

I have an issue where, after running for 10-15 minutes at nominal operating temperature, the coolant tank begins to spill coolant because of excess air in the coolant system. I've used a spill-proof funnel to burp the system as best I can, I put a flushing T into the heater core hose and flushed all the existing coolant out, still no dice.

I've done a coolant test strip with came back clean, all the coolant I drain when flushing it is clean, the radiator is new, the oil comes out clean... If there's a leak coming from head gasket, I'm not sure how else I'd be able to identify it short of doing cylinder compression tests.

I've seen some people joining the inlet and outlet hose for the heater core, and that fixes their problem. It seems somewhat common for these cars.

I am fully aware that it could be a head gasket issue, but I wanted to explore every avenue in the case that it's not. What else would you try on it?