One thing, even the core board has FCC the final product need to get FCC approval as well.
Making a core board, we need to sell a lot. It also takes a lot of effort as well. Making another board without core board may save some effort. That is two products vs one products. Not too much will be saved in certificates.
I’d rather to redesign the mifi totally:
Add a small screen
Easy exchange battery
Monitor battery from openwrt
Easier expend the interface
Etc