Which (offroad focussed) Garmin Device should you buy?
There is a simple answer - I would buy the Garmin Tread. It has the most features . I don't have one or used it. However, my offroad friends really like and most Garmin based riders in offroad events use it.
You only have to choose betwen the 5.5'' and the 8'' version. Depends on how much money you want to spend. The difference is significant. Advantage of the 8'' version (except for the screen) is that you can install some Android based apps like LocusMap or Drive Mode Dashboard to have the best of both worlds. However, you need to be a "techie" in order to be able to do that.
What do I use?
Mostly, I use an Android based tablet (less and less Garmin).
When I use a Garmin GPS (for example stil on my Berrt 1150 GS enduro) and now also as backup on my Husky 701 (for the Illyria Raid), I use a GPSMap 276Cx. Robust, I know how it works very well and still does the job.
I also have a Montana 700i (or 750i 🙈). It was intended to use on the Huky 701 as second device. I never used it, because in the Gibraltar Race and ATT, only one device was allowed (which is now for me an Android Tablet) and I found it too complex to use (i.e. I never invested the time to learn to work with the device). Anyway, if you want to buy it, send me an email 😉. It is not a bad device, with just never got friends (yet?).
276Cx on my 1150GS
Initial set-up for the 2019 Gibraltar Race (1150 GS enduro) - too complicated
Later set-up for the ATT (1150 GS enduro); Maps are standard OpenTopo