I have the 605 ( which I bought off mr mw 😉 )
It works perfectly for me.
To try and answer your questions…
The official Garmin maps aren’t OS but they’re ok if you’re following a route – for me, it’s as much about being able to see the contours as the trails themselves.
Yes, you can load a route and follow it. In fact you can easily do that even without base maps as I did for some time before getting the 605.
You can also use open street maps which despite the name have lots of offroad trails and is often better than garmin’s topo. Plus you can actually modify the maps yourself to add trails in via the osm website donuts getting even better each day.
I’d have no problem recommending the 605.