Ok, I understand where you are coming from, clearly that is the smart thing to do in your case. For me, one generation old means it has been tested and the bugs worked out of it. The newest Garmin is the 742, it is nearly twice the price of the older 741. The 741 has the same connectivity (which is what I was concerned with for future upgrades in network, Radar and Sonar), but fewer bells and whistles like the some of the sail assist (race start timer, etc) software.
This comes from my Garmin 741 Diagnostic log for NMEA communications:
The DPT sentence is straight from my B&G instruments, it has always sent this format (depth in meters). The previous Garmin unit (545) was able to read this and displayed the depth correctly. The 741 reads it and shows it in the log, but does not display any depth value (--.-). Garmin claims this is an 'old' revision of NMEA and no longer valid (they are probably correct). My solution was to read the DPT sentence with some electronics (specifically KBOX https://hackaday.io/project/11055-kbox
) and convert that to the DBT sentence you see above. The Garmin 741 likes that sentence and displays the depth now. The Kbox is a home grown NMEA multiplexer, it reads in a couple of channels of NMEA 0183 (different speeds 4800, 38400), it can operate on that data and then send it out in numerous formats NMEA 0183, NMEA 2000, Log to an SD card, out over USB cable or out over Wifi. It also generates a new open source format called SignalK - Signal K » Welcome
I run a signalK server using a RaspberryPi computer on the boat and all my instrument data is sent out over wifi in that format. It can be read by a free sharware chartplotter program called OpenCPN or by several apps on tablets or phones that display the data in numerical or gauge formats. With the exception of the depth sensor data all the 'hobby' electronics and such are independent of my boat instruments, displays and the Garmin chartplotter. They read the data and send it out, but for the most part my boat navigation does not depend on them. I do have a separate B&G depth hsitory display, but that display is slowly dying.
So I have my solution for now, but it Garmin thinks they can fix the DPT issue I would be happy with that too.