If the engine sucks and cant handle 40 fps at 1800p then drop all the way down to 1440p. Just stop using checkerboard because it straight up looks like a PS3 game in terms of image quality. Ive never seen a worse IQ in a game since before the PS4 launched.
It shouldn't be the CB. I honestly have no idea what's causing the issue but I mean, the first Horizon also uses CB and is one of the cleanest looking games of last gen. And yes, I know it's 2160p CB instead of 1800p this time around but other games that run at 1800pCB too (InFamous SS, Ghost of Tsushima...) have significantly better image quality.
Maybe the problem is the assets themselves, the game is full of fine detail, high frequency elements everywhere, which might've been created with native 4K in mind. Or maybe the AA solution doesn't work as expected for whatever reason. Maybe it's the combination of CB with DRS? It doesn't seem to be the case, since tech analysis says the DRS happens only in selected sequences and the IQ is pretty bad all the time, but who knows.
In my opinion the game doesn't need an VRR patch to smooth out the frame rate drops that it doesn't have (it performs nicely), but if only they could anable a native 4K@40 fps mode it would be much appreciated by the fans I think. Unfortunately that would only help those with a 120 Hz display, so...at this point I don't even know if there's any feasible solution to that mess. I mean, they've been trying for weeks patch after patch and as you say they only seem to downgrade the graphics.