Thanks for the info.

I bought the 3Gb, $9 Germany No Expiration plan. It does roam on AT&T as you described, and I am getting data service with LTE at around 30Gbps down, 4Gb up at my location in the Northwest USA. Ping times are 300-500 ms, but that's to be expected, and should diminish when in Europe I'd expect. It took a few minutes but I was also able to get IMS voice service working over the data from Bnesim on my iPhone 13 Pro Max. That has been a game changer for me since the 13 series iPhones will now allow 2 esims to work at the same time. An odd message appears in the secondary app claiming "You Don't Have Data Credit In The United States", but it is working with a manual selection of AT&T in Settings-Network Selection.

I am cautiously optimistic about this being another alternative for travel cellular services. I will continue to provide my experiences to this thread, and look forward to comments from others also.