I tried washers, after 3 on each bolt, it was just starting to touch the latch. I went to Ace and got some plastic 1/8" spacers and it now shuts normal, mild slam. I did not have to mess with the bumpers.
I have a 2020 Crew cab with the same seat back latch problem. I read of a possible fix by going to Ace Hardware and getting two 1/8" plastic spacer washers for the latch that is fastened to the rear bulkhead of the cab. Problem solved! Now there is no banging when I hit the brakes or go over...