Normally I don’t get into the 27.5/650b & 29er debate. I have always ridden 29er bikes (after 26, anyway). Just not sure whether as a hardtail AM rig, with aggressive trail riding, there would be any meaningful difference between the two. If I were to buy a full-sussy AM rig, I’d go 650b for sure.
I’d say the more riding along and up that you do to get to the trails, the more a 29er makes sense on a HT.
If it’s all about the downs, likely a smaller wheel is more fun, especially in skilful hands (mine aren’t).
The Stanton Switchback looks great to me. Also the Kona Explosive or similar.
But that’s all from conjecture not experience.