I've found a couple more recently tagged Sprint keep sites that are really close to existing T-Mobile sites. It'll be interesting to see when these get converted if they get everything or mid-band frequencies only.
I would caution against making that statement for everywhere. Personally I switched my line from Sprint to T-Mobile right after the merger completed and even then it was already a better experience and has only gotten better since. Also keep in mind that T-Mobile will continue to take from the Sprint network and redeploy on its network. Here they've already trimmed both Sprint's B41 and B25 spectrum. This will vary city to city though, but even if Sprint is still the better of the two networks now it's only a matter of time before that changes.
The iphones i believe will support VoLTE once they have the TNX sim.
As for getting an s21. Since it is a 5g device it will use the T-Mobile network primarily even on a sprint sim. The sprint sim will connect to non keep sites if needed. I would personally try to stay on a sprint sim on that device unless standalone 5g is important to you.