By the time you get your S20, it'll probably be preloaded with the update that defaults you to T-Mobile bands. Interestingly enough, the IP network part of the connection is still Sprint...but you get VoLTE across the board.
With that said, you can install Samsung Band Selection and pick which combination of bands (LTE and NR, though n71 will only work if you also have B2/66 enabled as well, and n41 for the most part doesn't exist) you'll let your phone on. T-Mobile band priority still applies to whatever you let through, so if you accept B2/4/66 you'll get those (probably even 12/71) but if you lock your phone down to Sprint bands that's what you'll get.
As an example, I've tended to select B41-only on my S20 if I'm in an area I know to have solid B41 (AKA where I live and nearby). That gets me significantly higher speeds than the default, which tends to sit on B66+n71. I got one roaming warning from this but latency and speeds are as if I was on the native network. Then if I head out I'll either turn everything on or select 25/26/41 as the radio in this phone is strong enough to sustain a Zoom video call where previously (Essential Phone) the only thing reliable was 1x.
The system isn't perfect, and sometime the phone will fail to lock on a signal at all after enough band selection changes, but a reboot fixes that. I have the band selection app as one of my quick apps on the screen edge app selector, so swapping things around takes maybe 15 seconds and a few taps.
It works now. It can take up to 3 weeks to enable it though after changing over, but usually it's within a week. It used to be a fully manually process, with someone having to turn it on, but it seems more automated now. You will lose VoLTE if your internet cuts out. Used take a few days to get it back (again, maybe a manual process?). But when I tested this weekend I got it back within an hour. Sent from my Pixel 4 XL using Tapatalk