Water pumps arn't usually a problem, thats not say it is a possibility. I still have my original water pump at 125K. If your headgaskets are not leaking coolant externally, which I doubt since you've already had them done then I don't think they would be the issue either. Coolant level dropping in the reservoir slightly is normal, due to evaporation.
My initial guess would be to replace the thermostat with a genuine Subaru one, especially if you still have the original one or if it has been replaced with an aftermarket one (they are problematic on Subaru's) and it isn't very expensive either. 50/50 coolant is the best to use. See if that helps.
My initial guess would be to replace the thermostat with a genuine Subaru one, especially if you still have the original one or if it has been replaced with an aftermarket one (they are problematic on Subaru's) and it isn't very expensive either. 50/50 coolant is the best to use. See if that helps.