Looking into a few things:
1) A proper shroud around the fan. Might also swap for Noctua Nf A9 x 25mm (zip ties are my friends) and shroud.
2) A slim fan behind the gpu, however, the asus tuf 6800xt is flow through and it does push heat through the back. Fan should help pull a lot more radiant heat out however
3) Install a Noctua NF A9 between the motherboard and PSU. Test as both exhaust and intake and see if it makes a difference.
Past that, I would have to look for a smaller gpu or different cooler. The main thing is isolating the heat from the GPU from going into the CPU intake.
I have a feeling number 1 and 3 will do wonders.
I would start with installing 25mm thick cooler on the cpu heatsink (N9 should be fine yeah) and that will push air better and also be close to the left panel, so you might not even need the shroud.