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In plainer english, I seal clearances were not an issue because that isn't a thing you can check. Regular valve clearance issues were not a problem due to them having been done several years ago. During disassembly I checked the clearances and found them all to be still well within tolerance, so I did not remove any valves. I just took the head off.

Thanks everyone, I guess I'll be taking it apart again to look at the rings. I used piston ring compressors and was super careful but it sounds like the consensus is a broken ring.

Whilst the head is off, turn it upside down and fill the domes with fuel. If the valves are sealing properly to the head when shut they shouldnt leak. If they do leak either they are not sealing or you have bent valves.
You didnt mention if you turned the engine over by hand before using the starter motor. 15 degrees out is a lot and if you turned it over by hand you would know if you valves and pistons were hitting.