Small update on this build.
Frame:
The frame itself is rock solid. The flexiness I thought was in the rear triangle was actually the rear wheel needing tensioning.
Got a few thousand miles on it and no issues to speak of.
Brakes:
Was having a strange issue with the GR9 calipers and replaced them with GRX. Anytime it would warm up outside (which is often in south Texas), the brakes would start dragging on the rotor. I could make it go away if temporarily if I squeezed on the brakes really hard, but it would come back in a few minutes. Shop tried bleeding and adjusting, but it kept happening.
Rear mech:
Also replaced with GRX 812 which handles the 11-46 cassette perfectly. The GR9 rear mech would easily get sloppy on the indexing. The spring was nice and strong, but it also made moving the bike on and off the trainer a hassle for me. Maybe user error. The clutch is a game changer for rear wheel swaps or moving to the trainer.