Both should be able to do what you want it to. The 520 is quite a bit smaller, which is one of your requirements. The 520 also gets you smart notifications (calls/texts) from your phone if that matters to you. You want live tracking, so seems like your phone will be with you.
I was wondering why they didn’t come out with a 820…but it seems to me that the 520 has bridged most of the gap as far as map functionality. If you want the bigger screen, you probably get the 1000.
Surely an 820 is in the pipeline. I own an 810, thinking the navigation would be great, but ultimately I never use maps. If navigating a potentially tricky route I was just as happy following a bread crumb trail on my older edge 500.
If I had to choose 520 vs 810, I Would go 520. If you totally need help navigating, there is always a phone.
If you are not travelling the 520 is what you want, I have been lucky enough to travel a few times (costa rica, california, alps, etc) with my 810 and it has been invaluable in those situations.
As far as navigation, I almost always ride in the same area. I would like, though, when I am doing a planned ride where they have cue sheets to be able to put in the route and not carry a cue sheet of if I want to try a few different roads, I can just put the course in.