What’s the difference between an unlocked Galaxy and the T-Mobile version?
The T-mobile version is software branded, the unlocked version is not. The difference might be Wifi calling and other carrier features might not be available on the unlocked version.
IIRC, one upside for T-Mobile is I believe the the code necessary for their WiFi calling is in the Android OS itself, so unlike AT&T who requires certification for each device to able to use WiFi calling on their network, it just works on any Android phone on T-Mobile. I believe it dates back to Android 5, but don’t quote me on that.
One aspect I’m not sure about though would be virtual voicemail.