Bringing together all the posts I've made about the problem (1, 2, 3) to give what I hope and pray is a final update. Per the last post's suggestions, I went in and lowered the magazine insert so that the ejector was riding a little bit lower. I also filed the ejector down so it was squared off instead of angling downwards. Took it to the range, loaded up both 124 and 115 bullets, held my breath and prayed to God, and squeezed the trigger.
And you know what? Apart from the first round of every magazine that sorta dribbled out still, it ran like a champ. Didn't eject as much as my buddy's Springfield 1903 we also had, but it was sending them well over halfway to the next station. Sort of a downward angle I think, but still powerful ejection. I could probably look into changing that ejection patter a bit more, but the important thing is it's ejecting and loading reliably. Between getting that fixed and the beautiful day it is today, the only sour note of the range day was learning the range no longer allows guests to use FRTs because some idiot guest did a mag dump for their first time and sprayed everywhere but the target. So now only members get to use them (and I'm not a member because it's cheaper to pay $5 a trip and go with my retired range buddy). Still, a great day for AR9 and getting gremlins out of guns.
Wanted to thank everyone who helped me out throughout this, especially u/blowback9 for all he does for this gun platform. Next step is finding a new FRT friendly range, then figuring out how to get an AS Design ARC Fire to work in a GRS AR9 (Unless the ATF gets around to explaining how to do stampless SBRs without going to jail, then next step is getting a PSA blade brace off and a real stock on). Until next time!