This might win the award for highest percentage of helpful and useful responses out of any POA thread.
I'd started quoting a bunch of posts to respond to, but realized that was going to be too cumbersome. So, I'll just do a more general response.
I definitely agree that one of the standard Japanese "big four" dirt bike offerings would make the most sense for a pure dirt bike in terms of cost, general availability, and then also my skill level. Off-road, absolutely a newbie. On road I'd still consider my skill level as "moderate." Definitely can outride some, definitely can be easily out-ridden by a lot of more avid motorcyclists.
The big things (and why I think I'm struggling with this as much as I am) is that I'm always looking for something that's unique, unusual, has some personality. Japanese vehicles (whether cars or motorcycles) normally just don't do it for me. Yes I know I loved the MU-2, that doesn't count.
I also really just don't enjoy 1-cylinder engines, the sounds etc. that they make. That said, I enjoy our little 1-cylinder go-kart that is stupidly slow (when it runs) so maybe for dirt bike purposes of bombing around, I can enjoy it enough anyway.
The adventure bikes have a lot more personality, As motorcycles, they're more up my alley. As far as the riding that I'm actually most likely to do, they probably fit most of it well. I think the real answer is probably two bikes, one adventure and one dirt. And so to that end, maybe the answer is I keep on riding the BMW and also look for a sub-300 lb cheap Japanese dirt bike, something in the 250cc or less category, that I can run around on and that maybe my wife can play with some as well more comfortably.
I feel like a used dirt bike is where I want to end up price wise for that, but also something where I'm not sure it's where I want to end up maintenance wise. Just due to the nature of what they are and how they're ridden, they seem entirely beaten up. Plus add in that I don't want to mess around with carburetors anymore, that really limits my options if I want to go that specific.
More to consider. I really should probably just do the normal Ted thing and pick up something that looks like it would work, ride it around some, learn more about what I want from there. All this thing means not enough doing.