Wahoo Elmnt owner here.
If you use the OSM mapping on RWGPS, the bridleways are marked, but they’re a tad inconsistent in how they’re represented (some are green dashes, some brown, some small dashes, some longer, some a mix). So, if it’s an area you know well, and you know which ones the bridleways etc are and you just want to string them together, it’s easy enough to do.
For off-road route planning in areas I don’t know, I use the Ordnance Survey site, then export the .gpx, and import it into RWGPS.
As for following a route offroad, the track is usually pretty well defined on the ground anyway. You don’t really need turn-but-turn: if you get to a junction, or you’re not sure, just glance at the GPS to show you where you’re supposed to be going. If you stray from the route it will start beeping and flashing red anyway.