Okay, so, having refunded the game on Steam, I decided to buy it again to try some more stuff. 2 hours really isn't a lot of time to try stuff before refunding, but I've got the game into a tolerable state now.
My first bit of advice is to set your monitor to a refresh rate, in Windows, of the frame rate you want to target. In my case, I'm targeting 60fps, because with a 5800X3D CPU and 4080 GPU, the benchmark suggests targeting any higher than that won't be smooth. So I set my monitor refresh rate to 60hz, and setting the game to 60fps with vsync then successfully achieves a locked 60fps frame rate. This is obviously a very very long way from ideal, as first of all it's inconvenient to have to change the refresh rate for different games, and secondly, the monitor doesn't run as well when it's at a 60hz refresh rate as it would at 120hz, it would be much better to have the monitor running at 120hz, and the game running at 60fps, duplicating each frame and giving better refresh characteristics, but in the absence of the game having the ability to do that, it's a tolerable workaround to run the monitor at 60hz.
My second piece of advice is to go into the gameplay and hud section of the settings, scroll down to near the bottom, and turn off the camera motion effect setting. That setting makes the frame rate look juddery even when it's actually smooth. It makes 60fps look like 10fps. Turn it off and the game becomes a million times better. This setting being on, and me not knowing it was the cause of the problem, was probably the main thing that made me refund the game first time around.
My third piece of advice is to turn the assists off. Even if you select the most expert option it will still have sport traction and stability assists, and these absolutely kill the game's physics. When you turn them off, the game actually has very good physics, up there with ACC IMO, and noticeably better than FH5, but with them on the physics seem very mediocre.
I still hate the fact we have to level each individual car up before we can use it properly, and the performance of the game is still absolute garbage compared to where it should be, but having done the above, I can at least enjoy playing the game.