Skyhawks are only spin approved when loaded in the utility category as you already know, and there is not much slack there. I take the junk out of the baggage area and leave my flight bag at the hangar when doing spins. An additional problem you run into with vintage Skyhawks is the seats coming loose at high deck angles & rudder kicks.
The newer Skyhawks do not have that problem (loose seats), but they have an even narrower utility category. I used to work at Cessna and I was told this is because the airplane has had a lot of changes, things like sound proofing and G1000. The 172S POH shows utility loading ends at 40.5 inches aft of datum, normal extends to 47.5. That's a big delta in an aircraft of this size. There really can't be anything behind the pilots and fuel needs to be reduced. I used to take the rear seats out of the back of the 172SPs as an additional precaution. That was a pain and added a half hour of fooling with seats to a lesson, so we did not do a lot of spins.