Just to clarify, you mean Boost would be de-prioritized, right? MVNOs aren't put on a different part of the network persey, they are given a different QCI. The lower the QCI, the higher priority.
Now it could be that some MVNOs are restricted to not access mid band or mmWave.
I've never really seen anything official, so take it for what it's worth and it may not be true, but Boost is reported to be on QCI 9 on AT&T, unlimited post paid is 7 and 8. Every thing I've seen has FirstNet at QCI 6. Below that is supposedly FEMA, DoD and other federal agencies.
Then you get into the whole topic of network slicing that Verizon and T-Mobile implement for Frontline and T-Priority respectfully.
Bottom line is outside of the First Responder and government agency specific plans you get what you pay for.