So, you want to start riding offroad, which device do you need? And in case of using a phone or tablet, which software do you want to install?
I think that it depends on the type of user you are and the amount of money you want to spend (overview May 2024):
Beginner, occasional offroad rider (and want to ride e.g. a part of the TET):
If you already have a Garmin GPS (BMW Navigator, XT, even the old Zumo 660), you can use that. Even if it is a road GPS. Just install a suitable map, get the gpx track and you are ready to go. And then, you don't have to invest a lot of money.
If you need a new device, you have two choices:
Buy a cheap (but rugged) Android phone, mount it, install Drive Mode Dashboard (if you are a techie, consider Locusmap) and you are ready to go.
Buy a Garmin device. Maybe you can buy a nice second hand GPSMAP 276CX or on old Montana (or an other road GPS). Otherwise buy a new one (which will cost you "some" money). Most devices will do. If you are road oriented, buy an XT2. If you want a more suitable offroad device, buy a Garmin Tread.
Advanced/Expert offroad rider:
If you want a Garmin, get a Tread.
For an Android device, get a DMD T-865 nav tablet, CarpeIter, WLPcom WT-7 PRO or special phone (i.e. DMD T-665 phone) and install Drive Mode Dashboard or Locusmap. Here next to having a rugged device, the screen brightness (>700 nits, but ideally more) and gps quality is important. That is why the number of suitable devices is limited.
If you want the best of both worlds then get an 8'' Garmin Tread. Because it is Android based, you can install next to the Garmin software also DMD or Locusmap on it and even link these to a controller
Rally use (i.e. Lamas Rally, Gibraltar Race, Alps Tourist Trophy) based on the Adventure Challenge Formula:
Garmin: Garmin Tread 8''
Android Tablet: WLPcom WT-8 RACE or WLPcom WT-7 PRO
More information available in the detailled sections about Garmin Devices, Tablets and Phones. Info about controllers is still under construction.
CarpeIter on the Via Del Sale on the BMW 1200 GS Adventure (2023)
For some proza on Garmin devices, please look here. And for my personal comparison on Android tablets, look here.
Other things to consider:
Why no Apple devices. Various reasons:
The software available is limited (No DMD, no full version of Locusmap), only Osmand, Maps.me and I think GuruMaps
The devices are expensive
They are not built for using long under harsh conditions. For example they get pretty hot when the screen is used for a long time (and might stop working).
Why not use an existing phone (that you use for day-to-day-use):
Safety - In my opinion, you should have a second phone with you in case something happens
Most day-to-day phones are not ideal in terms of specs (water and dustproofness, brightness of the screen, ..)
Why a special Garmin GPS for offroad riding:
Depends on your use. For an occasional and recreational rider, a regular GPS with OSM-based offroad maps is fine.
However, these GPS-es have more offroad features (e.g. routing for offroad), can show waypoint descriptions on the map (not sure about the XTs, but the BMW Navigator was not) and have better Garmin maps (which I don't use anyway, because of my own tweaks on OSM-based maps). These features become more important for rally Raid use and for driving in the mountains.