BT-500 Head Gasket Combustion Leak Test Kit - Made in USA

I would rule out head gasket first before throwing any other parts at it. Could be exhaust gasses making their way into the coolant, causing it to overflow. Rule that out first with a head gasket combustion leak detector: https://a.co/d/08b3qXDy Then if that checks out fine look at water pump or other components

Have your thermostat, timing belt, water pump replaced. You don't want to overheat a Subaru. You should probably do a block leak test of the coolant to see if head gaskets are going bad. That is a common problem. This is a brand I've used. https://a.co/d/0hV6LKWb