The key to success is proper cleaning of where you opened up the cracks before you fill them. I like laquer thinner applied with a small paint brush, then immediately blow it dry with compressed air.
I have used 3M panel adhesive which has strong adhesive properties and sands nicely. If you chose a filler which is significantly harder than the surrounding steering wheel material, it becomes difficult to blend the filler surface even with the surrounding original material. Sort of like trying to sand a rock next to a marshmallow.
I think I would use the 3M PA too. Filling the cracks might not help with the structural aspect of the wheel
if the cracks are throughout.
