Boroscope it ?
If possible ...
Leak down test it ?
I would do that just those and that would give you a better idea/baseline to know where to the car's at.
Burning any oil ?
That oil looks like Mobil 1 5w30 the way the additive package looks.
Just a friendly a suggestion to get your heads reworked by a competent shop. I'm in the process of helping my dad drop an LS7 long block into his Z06 right now. Dropped a valve pulling away slowly from a stoplight and cracked the block. Absolutely no warning, motor was quiet and strong up until it wasn't.Yeah we will see. I have a feeling the oil changes on this car wasn’t very frequent. Then again it has low mileage.
Who knows. The fun of buying a used car. They also have a bad cylinder head design where the valve guides fair and exhaust valve just drops in the engine and boom.