Thanks CraigB, I have found a workaround online that seems to have (semi) fixed it. Have put in around 12 hours now. I'll leave the fix here because it seems to be a problem for many.

First, open game install folder -> Redistributables and run the exe's. (even on win10 my DirectX files weren't up to date.)

Next, do this - https://support.rockstargames.com/articles/360040482073/Red-Dead-Redemption-2-on-PC-crashing-with-ERR-GFX-STATE-error

Now the main 'fix' (as just doing the above alone didn't!) is to open save folder (usually 'documents->rockstar games->red dead redemption 2->profiles) and delete all files starting with SRDR except the latest/highest numbered (which is the autosave).

Unfortunately, I have to do this every time before I start the game AND deleting the SRDR files deletes all manual saves. But at least I have had zero crashes while playing.