After looking further into it, I found that Garmin specifically lists the GPSMAP 7xxx series, which includes my 7212, as compatible with the AIS 600. However, I have not found the 7212 listed on Garmin's AIS 800 compatibility page.
Bob's comment about future compatibility between newer AIS units and older plotters also gave me something to think about. Before spending $1,100 plus tax on an AIS 800, I contacted Garmin and was told it "may" work with the 7212, but they could not confirm it is officially supported. That makes me a little hesitant to spend that kind of money.
For now, I'm going to continue looking for a clean AIS 600 since it was designed during the same era as the 7212 and is known to be compatible. If anyone has a complete AIS 600 with cables and a resettable or unprogrammed MMSI that they'd like to sell, please let me know. And yes my current system is NMEA 2000.
Thanks again for all the help. For a retired guy, it seems hard to justify spending $1,100 on a "maybe" when the difference could be put toward diesel, crab bait, and a respectable gin ration.