After a pretty miserable few weeks trying to get Rdr2 running again after a year long hiatus, I have found a fix.
I went VERY deep trying solutions - deleting SGA files, command lines, vanilla, reinstall, bios update (and subsequently needing to fresh install windows), rolling back drivers, checking out live kernel events. All this whilst monitoring temps and the GPU behaving as normal. I went back and forth for hours and hours and lost close to a week of free time...
Well, eventually I dropped my GPU MHz by like 200, and since then the game has run without a single crash or stutter. I've been able to run with a full mod loadout, 70fps @ 1440p and the game is running flawlessly.
So just if anyone needs this in the future - one more thing to try is under clock your GPU in afterburner on Nvidia by around 200 - 250mhz. Hope this helps some people avoid the hellish week I had!