Heeded Tim R's advice. And double checked with SH & Garmin.
All done, working well.
Sending the port expander back since I won't be adding any Garmin network devices. Just replaced the depth display, wind won't work with my 3208, XM is too rich for my blood and so is a second plotter.
Radar works fine direct to the plotter.
Now that I finally understand some of this a bit better, I'm going to feed the old Ray ST6000 autopilot to the chartplotter to make MARPA work.
And, I found a second VHF cable behind the nav station. Plugged it into the old VHF and found the other end at the base of the mast. Plugged in an emergency VHF rubber ducky style antenna and presto, spare base station! Yea, limited range I know. I'll be testing it soon to see how limited it is. Like to use it for bridge tenders so range doesn't need to be extreme.
On to the next project...Lectrasan electrode! Oh Cr@p!