On the mod front I'd expect things to go similarly to GTA V. Rockstar locks the files down as best they can, modders eventually find a way in.
I forget how long it took for GTA V mods to start appearing. Some number of months?
RDR2 is a less interesting game for mods though, just inherently less stuff in it. It's compelling more because of its atmosphere and narrative than because of its sandbox elements. I'm sure there's some fun stuff mods will eventually allow you to do, but I'm not immediately compelled by the idea of buying this version for that reason, unlike GTA V. Wait and see on that.
My 970 may or may not be able to outperform my PS4 Pro with this game, but regardless I don't really find prettier graphics to be much of a reason to buy and play a game a second time. After 30 minutes you'll have gotten used to the prettiness and are just playing that same old game again. And I'm not particularly interested in doing that after having spent like 150 hours on it. I rode until I couldn't no more.