The maps on the device are great if you are a road cyclist. I do a lot of MTB and had a great OSM IMG file on my 520 (without nav obviously). Is it possible to load this on the 820 and select it on occasion? Couldnt see in settings.
I loaded an OSM map for my area into my Edge 820 and it shows up in all of my activity profiles and was automatically enabled in all of them. Check in Home → Settings → Activity Profiles → <profile> → Navigation → Map → Map Visibility. For me the list of maps there includes the new OSM map I added. You can disable (or enable) maps on a per-profile basis, but I didn't spot anything to globally disable one.