So I've tried the MTX Connect eSIM I mentioned above. Super easy to setup an account online and order a SIM; cost EUR 5 and includes an EUR 7 credit (Chase denied the charge at first but approved after I responded to their fraud detection).

Super easy to get the eSIM working on the iPhone. I then downloaded their app which has a nice interface to manage your eSIMs, topups and data packages. You can use Apple Pay to add to your balance. I have the 7-day Free Chat since it's a new SIM, and that worked well (roaming on AT&T in my area). Their support has been pretty responsive to my inquiries. I feel fairly confident this will work well, as advertised, when my son goes to Costa Rica in April.

Their pricing is very simple and covers their global roaming area, so a nice option for travelers hopping through different destinations too. One can perhaps find a cheaper option for specific local regions, but overall this seems better than similar Flexiroam and Gigsky packages.