just serviced my mates bike & it took me all my might to undue his rear qr.
personally i do mine untill i feel a little resistance at the half way point.
can you actually over tighten a qr, he is a large hefty chap so his tight & my tight are probably way different.

is there anything that could get damaged by over tightning ie hub bearings.frame or anything.

Cup and cone hubs or cartridge bearings? You can compress the axle slightly so could in theory overtension cup and cone hubs. A good cartridge bearing hub wouldn't suffer in the same way though.

To me, tight has always been governed by can I undo it again? As you say, normally, you feel the cam action kick in at 90 degrees. Or, another way is it leaves an impression in your palm for a moment. Too tight and you might snap the rod or strip the threads out of the nut the other end.

they are cartridge bearings.

Then his only risk is breaking the skewer or nut

as above should leave an impression on your /palm and be capable of being undone without a lever!!!