The fallback priority scheme you want should work. The RMC from UDP stream will be ignored, as it is lower priority. This is not shown in the debug window, and perhaps it should be indicated somehow to clarify the action of priorities. I'll add this to the list of ttd.
You can test it by opening the NMEA
debug window as in your screenshot. All should be green as shown. Then simply unplug the GPS on comp1, and watch to see in you get a priority switch message (in red) in the NMEA
debug window. It should switch to UDP.
I'll try to simulate this config if you have trouble with the test procedure.
Meanwhile, there is another complication which I just noticed from your screenshot.
Your ownship position is also being reported by the AIS
transponder in the AIVDO sentence. This could be the source of the jumping. You really only need one position source from UDP, but I see three
(RMC, GGA, and AIVDO). RMC and GGA will be ignored due to higher priority duplicates from primary GPSD stream, so that's OK. But, if lat/lon from AIVDO does not match the lat/lon from primary GPSD RMC in normal operation, then you will get jumping.
Solution: on UDP stream, filter out AIVDO.