I have the same problem with wanting a second wheelset for my fatbike.
There are no off the shelf 29r wheelsets that fit my 26" Fatboy spacing so I need to get a custom wheelset.
Costs as much as a half way decent used FS 29r mtb.
But, think of it this way. You have a nice bike, an investment in a second wheelset means it becomes very useful and saves some space, if that matters.
I want the second wheelset for my FB because i have way too many bikes as it is. My long term goal is to create three bikes to cover most everything:
Hardtail fatbike with 26 and 29r wheelsets
All roads bike with gravel and road wheelsets
FG.