The edge 800 was released in 2010, so 13 years old. Personally, I would find a way to get a newer Garmin and update MacOS. There have been a lot of updates & improvements since then. At some point, tech companies need to drop support for their older devices in order to better support the newer hardware and software. I know it's not the answer you want.
It should carry on working. Only if you need actual Garmin Support or try to update the GE version would it be a problem. Keep using the Garmin Express version you're using now.
