810 oddly doesn’t have GLONASS satellite (Russian GPS) support. Not sure how much improvement it is, but I’ve got it on the 510 which has GPS + GLONASS. Supposedly quicker fix and more accuracy which was one of the reasons I went for it. Not sure it is more accurate though, but the fix is very quick usually.
510 though has no maps and turn by turn. It does have breadcrumb trail to follow but not that easy to use. Otherwise it’s much like the 810 under the hood.
1000 has funky things like WiFi.
Most Edge models with ANT+ can do HRM.
Communicating with an iPad, you’ll need a model with Bluetooth (510, 520, 810, 1000), and the relevant Garmin Connect app, then a lot of patience trying to get it to pair. Alternatively the WiFi in the 1000 I think can also connect to the app which likely is more reliable.