If you want to eliminate body flex, these are some of the products you might want to consider.
I will be placing orders now and the ETA should be 10th of October.
Cusco Rear Bumper beam - £110.40
https://www.part-box.com/965492re-cusco-rear-end-bumper-beam-power-brace-toyota-gt86-subaru-brz?search=965492Re&description=true
Installation guide here
Cusco Rear Pillar Brace - £132.48
https://www.part-box.com/965492rp-cusco-rear-pillar-brace-bar-toyota-gt86-subaru-brz-not-harness-bar?search=965492RP&description=true