Looks good! However, when I allow iframes to be set to maps.trom.tf, it still gives a connection refused error. The content security policy headers of maps.trom.tf don’t look that restrictive at first sight. Anyway, will need to explore this solution better as well. Will do that sometime later this year due to time management.
Done! We’re now using uMap, which is one of the most popular custom layer maps for OSM (OpenStreetMap). I find it important that a project remains active for a long while. I’m not sure if the TROM cloud services fall under that. But we can always switch at a later date, exporting the data is possible and probably can be imported quite easily somewhere else. Open standards for the win!