TIL that there's a utility called kio-fuse which just completely solves the "cannot open files from remote filesystems without copying first in KDE" issue on #NixOS
Like, you literally just have to add it to your systemPackages and that's it, now it all works and it auto-generates FUSE mounts for stuff instead of expecting every application to support KIO explicitly (which of course only KDE software does)
@ZeroEcks It needs to be brought in manually; it does not seem to get enabled by default with Plasma enabled (I've created an issue about that earlier today because it probably should be)