FWIW, unless you’ve relocated, I think we’re both in the same part of the Tampa Bay Area and have similar coverage concerns. I switched from AT&T to T-Mobile about two years ago and couldn’t be happier. Our family plan has 5 lines and my Apple Watch and it’s nearly $100 less than we were paying on AT&T, all for Unlimited everything. To top it off TMo also pays for my Netflix.
If I understand correctly, unlike AT&T’s requirement to approve it for any device in their network, TMobile’s voip and WiFi calling functionality is coded directly into Android since Android 5.0, so any phone with compatible radio hardware will automatically have that on TMo.