Thanks for the detailed feedback. According to one Immich dev, they used to use OSM’s raster tile provider but switched away from it since they were causing too much load on OSM’s servers.
There does not seem to be any non-commercial vector-tile provider at the moment (though OSM seems to be currently working on it), and it seems really overkill to try and self-host a tile provider (at least with the default level of details). Maybe the way is to find a balanced level of details that makes it reasonable to self host