One of the nice things about 2 MFD and a NMEA 2000 network is that if you lose one display, all the stuff can be displayed on the other display, redundancy. If you lose the display of your stand alone depth gauge, your out of luck. With a transducer feeding the NMEA 2000 network, depth can be displayed on either MFD. Add a second transducer and you have complete redundancy. I have 2 transducers and several displays. No need for a lead line.
When you're out of sight of land, and your GPS goes down, do you plan to use a radio directional finder or wait for dark and use celestial navigation?
For a boater who has such a dim view of modern electronics, you seem to have an awful lot of faith in an electronically controlled outboard. Have a backup pair of oars?
My radar would pick those markers up at 100' on a flat calm day, but then it's 6kw.
Ted