The WR250 R yammie if you have lots of money, or the 250 Kawi if you want cheaper, or the little Yammie TW200 looks like fun. Or something in the 250 family from Suzuki as well.
I think 250 the recipe. The 400cc and 650cc family starts to get heavy, bulky, and spendy. Sure, there are exceptions, I just dont think the kind of boonie riding you are talking about requires lots of CC's. I have the same quest as a second bike. My KLR too piggish for some terrain.
Just the three that pop into my distorted mind. A KTM or Husky certainly great choices too, but... $$$
it's largely "ride what ya got".... but constantly picking up a bigger bike gets old..