Also I just checked a bit online, and it is a model A1863
Okay, this kind of simplifies things despite having both CDMA and GSM support. Trying to get Sprint MVNO support usually requires that you have an existing Sprint LTE SIM card already paired with the phone, plus, Sprint is the least of the big four in terms of native network coverage and network reliability, roaming agreements or not, which makes them very hit-and-miss by region - so, let's just take it off the table.
T-Mobile's LTE coverage is actually pretty decent, nation-wide speaking, and especially in metro areas.
Verizon's got the most native network coverage of the four, followed only by AT&T.
AT&T MVNOs have the "must be Apple blessed and approved" to have MMS working, which simplifies your choices.
There's also the poor antenna design issues with the CDMA iPhones, and the fact that they do better with 700MHz block spectrum reception over 850MHz. That places a minor bias towards Verizon (and surprisingly T-Mobile LTE) over AT&T anyway.
Network speeds are really a non-issue unless your demands are unreasonable. Given you're only using 2-3GB of data on an un-optimized iPhone, I doubt that's really an issue. Outside of insanely bloated mobile web pages and trying to stream video (which your target usage level can't handle anyway), nobody is going to notice much difference past about 4Mbps anyway. Faster access just means you're likely to blow through your data cap faster, anyway.
As for call/text levels, you really only have "unlimited" plans with 2GB+ data availability. Just note that "unlimited" usually translates into around 1500 minutes or so a month with most MVNOs, which is around 25 hours of talk time, but it's more of a soft cap than hard. If you're looking at needing to be on the phone consistently for 30+ hours a month doing business, you need to go with a postpaid plan.
Really, all three will probably mostly be just fine for your needs, unless you plan on wandering out into the middle of nowhere on a regular basis and demanding near perfect reception while doing so (geographically speaking, usually the non-city/interstate parts of 40°N or higher, between the Rockies and the Mississippi, but is hardly all-inclusive - the hollers of Appalachia come to mind, as well as most larger rural farming regions). Verizon postpaid with roaming agreements would be your best option under that scenario.
That really only leaves one question. Do you want brick-and-mortar access to SIM cards and airtime or not?
If you don't care about physical access and have a way to either get the SIM card to you before you arrive or mailed out to the first place/people you see after getting off the plane, go Red Pocket. Given the prices are identical between their T-Mobile and Verizon plans, just go with the Verizon SIM card if you want better coverage, or their T-Mobile SIM if you want international SMS support. Even though Red Pocket and US Mobile have the same prices for service, Red Pocket also provides outbound international calling support with these plans, though it isn't direct-dial.
If you want direct international dialing and SMS support, go Google Fi (T-Mobile with roaming agreements on Sprint/US Cellular).
If you do want physical store access, going Walmart will be the lowest common denominator. That, in itself, is a choice limiter.
And with that, I don't think I've much left to add to the discussion. This is kind of reminding me why I stopped doing the guide in the first place. The industry has become too much of a fragmented mess to keep up with.
