Hmm, expected alot more input by the community on this one. Hope reverse proxy is high on the list for currated apps, because it helps manage accesss to them all.
Managed to fix the problem. Go into True NAS interface from Settings in Hex. Go to 'Apps' menu, click 'Configuration' drop down button, click 'Choose Pool', select your storage pool then click 'Choose' - the app service will then start, go back to HexOS and install Immich again and all will work.