US Mobile gets you QCI8 (same priority as Verizon postpaid) when you're on the Verizon network with a 5G device, and they let you pay for QCI8 on AT&T.

USM is the only MVNO I've seen that actually advertises QCI tiers. I had to look the term up when I was initially considering them, as I'd never even encountered it before. It was a major factor in finally feeling confident I wouldn't be giving up too much by leaving AT&T.