You might want to take a look into something such as the Salsa Fargo or Cutthroat - fantastic options for what you are looking for.
I have a 650b gravel bike and a 29er MTB.
The 29er will take a bit more effort to get up to speed, or to slow down. it doesnt feel as tossable, purely from a wheel size perspective. Generally speaking, 29er motivates you to stick to a desired speed, and hold that speed over a variety of conditions, as efficiently as possible. once holding, it will navigate over most obstacles your way with very little penalty in momentum.
I would prefer 27.5" on my MTB because I would prefer the extra acceleration/braking response with a slightly more tossable nature.