Regarding a back-up for GPS/GNSS this was posted here in 2017
Transas ECDIS allows bearing and ranges obtained by visual bearings or radar to be plotted on the ECDIS. Seems to me that is the way to go, along with such methods as parallel index lines on the radar as well as visual monitoring
The subject of deck officers learning visual navigation using deliberate practice has also been mentioned a few time here.