I can see why you might not want to do it in the TDM since parts don't seem easy to obtain there. No idea whats easily available in the US market.
Parts that are specific to the TDMs are 3 days away by special order, best case scenario, if you're near a city. They stock virtually zero of them - all shipped as needed. I have no clue if they are still making any of the specialized parts for them, or we're slowly draining the stockpile they started with years ago. For sure some parts are no long available, from anywhere, best I can tell. Fortunately, most are.
That said, my bigger issue is the rear seat space (w/ the bike fully packed) and lack of electrical (to run heated clothing). Last year in July we road through 35 degF conditions w/o heated vests and it was borderline. My GF damn near froze off the bike. I had to pull over and thaw her out and buy her a pair of ski gloves. Bottom line, some of the places we ride and want to ride get damn cold even in the peak of summer, and we also like to ride all around the west coast during the winter which is typically in the 50s or cooler. I'd love to get a bike with a huge electrical output capacity (GSA = 720 W which is pretty hard core).
I'm not trying to convince myself of anything. I simply can't think of what other bikes meet those specs. The off road part is really what makes it tough, as there are plenty of awesome full touring bikes around, if the goal is 2 up comfort.
If it's any consolation, I already have a BMW and still ride my TDM all the time. I love the bike, and will keep it until one of us is toast.