I had an r1200gs and wanted to do the same. I sold it and got a G650 x-challenge. (Single)
i went from 200kg dry to 140kg. had a great time with that bike. Learned bout off road riding. Even toured on it.
The x-challenge was great off road. Good suspension and I dropped it plenty of times without damage.
I rode it places you could not take a 1200 or 800.
I now have F800gs. Mainly because i wanted to do a bit more 2-up touring,
I am not planning to take it on the same trails.
So.. Keep the 650 twin or the 1200 and get a good 650 to learn on and still get some gravel roads in. It will hoan your skills for the roads you will take your other bikes on.
p.s. I recommend something reasonably modern. Upside down forks and plenty of travel.