Decided to return the monitor myself. The flickering was just too much on a 3090, very noticeable when fps is varying a lot, especially between 70 and 110 fps. Newest firmware, even bought a £20 fancy Display Port cable and it did nothing.
It's a real shame because the colours and black levels were excellent. The other thing I disliked was the smearing effect, which made games look like they were running at a lower framerate than they actually were. Locked 144fps on my old TN definitely felt a LOT smoother than a locked 144 on this monitor.
Unfortunately I'm just going to bite the bullet and go for a higher end panel - the LG 34GN850.