The pebbling at the edge looks like a bit of detonation and that definitely does not look
like a piston running at a true 12:1 AFR. I've seen lots of much darker tops on road race Corvette pistons that have run up to an hour at a time at 13-13.2:1..
Is that the relevant wrist pin to the left, appearing a little cooked?
How do the bearings look, especially the rod uppers?
X2 a bottom view of the holed piston as well as a couple of the cracked ones.
How thick are the piston tops?
Any pinched rings?
You'd have mentioned it if you were running boosted, yes?
Presumably there is no knock sensor system on the engine?
Any idea what is peak torque RPM on this engine? That would probably not be a good range to run for endurance.
I don't know what MBT spark might be for that engine, but 28° sounds a little on the high side. You might do steady-state dyno testing and develop a spark table for a couple of percent retard from MBT.
If you're stuck with that piston p.n., thermal barrier coatings would be a cheap precaution. If they aren't approved, Swain for one has an invisible "stealth coating" available.
Piston squirters are more work but if legal would help substantially if other avenues prove to be dead ends.
Felix, qui potuit rerum cognscere causas.
Happy is he who can discover the cause of things.