Not sure if a MTB fatbike would be the right choice. I think a solid dropbar touring bike might work better for mixed terrain. There’s a reason why gravel bikes are taking off. It’s precisely for this very reason. Take a look at some of the touring bikes by Surly and Salsa. Also, take a look at the Marin Nicasio. I personally have one and have been quite pleased with it. Funny thing is that Marin just put out a version called the Nicasio+ which adds 650B wheels with WTB Horizon tires. I put the Horizon’s on mine, which to me are perfect for both road and mixed surfaces like sand and light gravel.