I have a Garmin Edge 840 bike computer and I do like it (more than the Wahoo ELMNT Bolt v2 I had before), but man some of the design decisions are just baffling to me.
Like why can't I have tones for my notifications (for refuelling), without also having them for navigation? And why do notifications close themselves after 5 seconds? I always miss them because I either have the tones silenced and happen to be looking where I'm going instead of my head unit when they pop up, or I have the tones on I miss them because the navigation tones are so obnoxious I just learn to ignore all the sounds.
And why can't I disable the virtual partner, and why the hell is the arrow on the map for the virtual partner bigger than the one that represents the actual rider?
/rant
The 840 uses different firmware to the 520 and you can't disable the VP at all. You can however access the file system as you described, so my workaround was to edit the XML files for the map themes and make the arrow for the virtual partner 100% transparent. The VP is still running of course, but that does fix most of my gripes with it.
I'll have to check if I can do something similar for the sounds, but I'm not sure it will be possible since it's just a beeper and I don't imagine there are ordinary sound files that can be edited.