Categories in motorcycle business are all over the place in recent years; in order to boost sales the industry will use whatever classification is currently most popular, that's why you see bikes that are 90% road bias sold under ADVenture umbrella since like you said "everybody want's to conquer the world "on GS and in Klim style" (i read someplace the GS won't even start unless you are wearing Klim)
Everybody wants "one bike for all conditions and terrains" solution, but that usually doesn't work in real life, and even if the bike is equally good on pavement and gravel, tires are a limiting factor, and you always need to ride asphalt to get to gravel (and vice versa).
If you can afford a second bike beside your FJR, there are many bike choices to explore some "off road" trails, depending of how serious you want to get in the dirt. I was always ridding road bikes, and this summer picked up a '18 Vstrom 650XA to get a taste of gravel, and happy to report this bike does excellent job on both pavement and gravel. So for the price it was a no brainer, and a great entry level bike to get some experience "off road" without breaking the bank. And from what I could see when ridding gravel trails this summer, there are not too many of the big (GSA's) playing in the dirt.
Once when you get better off road (and if your knee will allow), a natural upgrade would be bikes like Afrika, Tenere700, Tiger's, KTM's etc and all the way to GS.
I researched soo much and like always, the more you look, the less you know...so this bike came up for sale, liked the color, liked the price and since I read a lot of positives about the Vstrom's, I picked it up, didn't even test ride it. Based on my limited ownership and 5000 km I would definitely recommend this bike to my friends.
Good luck with your "new knee" recovery and with your second bike whatever you choose.
more pics here: