First I would try pressure testing the cooling system. You can have a shop do it or you can buy a pressure tester (not cheap). Any external leak will be evident with the system under pressure. System should hold 15 psi without dropping more than 1 psi.
If that checks out ok (like it did in my car with a BHG) then you could try doing a block test. Cheaper than a pressure tester about $50. Will tell you if there is exhaust gas in the cooling system meaning you have a blown head gasket, cracked cylinder head, cracked cylinder or cracked block. If the fluid even hints at turning from blue to yellow (like green) thats bad.
You will find out if your head gasket is bad with these tests. My opinion is you just have an external coolant leak, usually these cars like to top off and blow coolant out of the overflow bottle along with random overheating when the head gasket is bad.
Good luck!
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