Any reason you didn't consider Google Fi? I've been pretty happy with it.
Google Fi can also do cellular over WiFi if your phone supports it.
Google Voice now supports data calling as well, so you can have same end result of WiFi calling even if your carrier or phone will not do voWiFi.
I don't see the appeal of Fi at all.
So you have data SIMs that share from the same pool? I use a few hundred GB's per month. How much would this cost on Fi? I currently have 4 unlimited SIM's, a extra voip number, and a low speed SIM with one of my line numbers copied. It costs about $160/mo. If I run into an area where I don't have service, I have a second provider that's also unlimited and costs $4/mo.
Sprint is weird, since they really only have halfassed voLTE they rely on a giant legacy CDMA network that should have been put out to pasture 5 years ago. But they do have the largest number of roaming agreements if all you care about is ability to call (and not have data). They do have fast data but it requires you to be in line of sight of a tower, not have walls or wet leaves in the way, and have a phone supporting HPUA so you can actually upload things.
AT&T's network has basically zero investment in recent years, unless in Georgia or a handful of other markets it's the worst of the 4. Poor spectrum planning, low capex, the only reason they are #2 is because they keep buying up companies with customers (shitty Mexican carriers and DirecTV).
Verizon still seems best (hardware, spectrum planning, roadmap, coverage), but no plans ideal for my needs. Poor for travel.
Three UK will let you roam to the USA and is $8 per month. $8 is the total plan cost, and there is no supplement for roaming!
I use a Verizon reseller (Total Wireless); it works great and is pretty cheap for the US.
Travel is not a problem with Verizon: Verizon phones are all unlocked, and have SIM cards. Just buy a prepaid SIM card when you're out of the country and use that for data. It's generally much cheaper than any service in the US; on my recent trip in Europe, it cost me 15 Euros for a 3GB SIM card, and I don't think I used half of that in my 2 weeks there, even though I was using my phone extensively for navigation and more.