> I still kick myself for spending 6x MVNO pricing on my cell phone plan with a legacy carrier whose features I didn't need.
I have a friend who tried to switch to a MVNO (Cricket, I think) to save money and immediately switched back. Even though both companies were on the same network, the MVNO customers must have had a lower priority, because their service level was noticeably worse when literally the only thing that changed was the SIM card.
There's a good reddit, i think NoContract, where you can go to learn more about MVNOs. There are several tiers of them in practice and they each have their own "catches" and "advantages". I used Cricket many years ago when they had a punishing speed cap. In the modern days some of these caps have been relaxed, but as you suspected, prioritization is the main way the actual carriers differentiate themselves from the MVNOs that sell access to the same towers. The worst MVNOs have terrible priority and in any well-populated area congestion makes them super slow almost all the time.
The thing is, this is highly variable -- and also geographically variable -- and some MVNOs can now offer similar priority as a mainstream plan. US Mobile is one, which I've been using for a couple years. Their neat advantage is that they will sell you a SIM (or e-sim) that rides on your choice of the big 3, and they'll also let you port between them without any other change to your account. They call this "Tele-Port". Some people will do that even just to go on a vacation to a state with different "best carrier", since there's nothing stopping you.
Not all MVNO are the same in this regard, some sell the same quality of service data tier.
The only time deprioritization has been a problem for me is when I've run out of data on my limited plan. With the major carrier, it was still usable; with the MVNO, it was not. As long as you stay below your plan limit (or, for those on unlimited, don't try to tether and use hundreds of gigabytes a month), it's essentially the same service.
I switched from T-Mobile to Google Voice a few years ago for this reason. With 5 lines on the plan the T-Mo version was way too expensive. But then Google Voice raised their prices and T-Mobile offered as much better multi-line discount and I ended up switching back. Also, Google Voice tech support is absolute dogshit.