First thing I think of is too much chain tension. I've seen so many single speed chains that are way too tight. One can sometimes feel each link engage each tooth when on the repair stand. This will accelerate bearing wear big time. The right chain tension is no tension, as measured at the tightest point of the chain's rotations. Andy

Forgot to mention the type of bearing- Use the OEM design/spec if at all possible. Angular contact bearings are very dependent on the ability to preload them axially. Think of the cup and cone bearings that are used on inexpensive rolling things (baby strollers and joggers as example), without any axial preloading these wheels have a lot of slop and rattle about on the axle as the bearings wear. I strongly suspect your (brand yet unknown) driver has zero ability to address this preloading need. Additionally double row and/or angular contact cartridge bearings are often wider than a single row radial contact design on the same OD and ID.

Last bit I'll add is that of the dozen+ freehub bearings I have replaced (cartridge radial contact) almost every one has been a royal pain to remove, heating the FH body in the oven to 200+ F helps Andy