Are you able to post pics of each piston? ( are the casting numbers / general shape the same on all? )

Got it on the 4 mm thickness of material.

I'd do a few things here:

It would be a good idea to cover the outer skirts with tape during grinding. Be sure to weigh the piston pre / post tape then calculate your target weight, don't try to remember it as target with tape will be more than desired bare piston.

Make sure of piston weights.

On the heavy ones, remove the casting boss on the left side of your last pic.

Increase the OD of my green cut but only in an area that does not enter or reduce the profile of the skirt ( the red area ) This can be done by hand with a die grinder. The center of the red area is what keeps the piston from rocking left to right, as you travel towards the pin the loads are taken up by each end of the red area. The pin area does nothing for front to rear location.

Remember you only need to get within 10 G not 0 if enough material to remove can't be found.