How did the height feel to you? Obviously the ST won't be as easy to pickup as the DR and just a little easier than the FJR.
The riding position might bother you, it does me over long rides and like you I really need to lean forward a bit (history of back issues too). I actually have my bars, without risers, leaning forward a bit to try a gain that bit of lean, but I'm still in slouch mode though.
I also had FJR's before the ST and I'd like to get another one, a 2016 or 2005. I like the ST but it's not as comfortable as the FJR on long rides and the wind protection is not as good IMO. Saying that the ST is fun, like a big dirt bike, upright viewing of the countryside, handles great, feels light, and roomy like the FJR, I feel mine is just not as smooth as the FJR but it is easier on tires and easier maneuvering in tight situations.
I'm in the South and ride year around so the FJR did get hot on summer rides but felt great in winter. With the ST its easy to remove the hand guards and optional "wings" to help cool things off. So it's "convertible" between seasons so to say.
I did very little off road riding with my ST, at 5'7' 180# and 62yrs of age its more than I want for trial riding. I have recently lowered my ST and make it a point to see very few dirt roads or cow trials. Personally if I were to swap off I would keep the DR but it would be nice if you can get more seat time on a ST before making the move.
Good luck.