I am thinking about replacing my Garmin 200 (which is great if basic) with something that can display maps and have been considering the models you list.
All the Garmin units you mention can display third party maps as detailed here.
http://www.dcrainmaker.com/2013/05/download-garmin-705800810.html
I have ruled out the 520 because of limited memory and the 810 as being too old (released 2013 and so maybe due for replacement) while the touring seems to lack some of the fitness features I would like.
I am holding off for a while in the anticipation that an 820 will soon be released which will have the new features of the 520 plus support for micro SD for map storage. No announcement from Garmin yet but from forum posts it seems that many people are expecting it to arrive reasonably soon.
HTH