Thank you for your thoughts everyone!
Until someone demonstrates otherwise, Verizon and Sprint are non-starters. The way to go is with either T-Mobile or AT&T or both (via 2 sim cards).
Also today I found this helpful page on buying prepaid SIM cards.
So now I just need a modem that supports both T-Mobile and AT&T, and then I think I can buy a SIM card for either and see how it goes with that, … I think.
Looking up AT&T and T-Mobile in this Librem 5 carrier compatibility thread: seems to show pretty good support using the BM818-A1 modem. Only bands B14, B30, and B71 are not supported and none of them seems like a deal breaker. The other modems all have many significant missing bands.
So it looks like the modem question in the US for AT&T and T-Mobile is answered best with the BroadMobi BM818-A modem.