Hi Chezzy. Why exactly do you need the union? You could have radarr/sonarr import directly to the mounted remote if you allow it to be read/write. To do this, I would use --vfs-cache-mode writes and supply a --cache-dir that is on the same drive as your downloads folder. In this case, the mount will appear to have all files even though some are in your gdrive and some are being uploaded/waiting to upload in the cache directory.
Alternatively, if you want to have a read-only mount for Plex, you could use either two mounts or two unions, one read-only, one not, and point Plex to the read only mount/union.