Textures on ultra when you have the VRAM because it doesn't really cost any performance but you want the full texture details.
AF on 16x is a no brainer: better textures with no cost.
Lighting and global illumination quality: doesn't really change anything in terms of visuals and performance-wise. It might give a slight boost.
Shadow quality: you want that on high. On medium, it gets blocky and the "refresh-cycle", e.g. from moving trees, is lower and shadow movement looks blocky and not smooth. Ultra really does stomp your performance, though.
Far shadow quality: you want medium at least, since low removes shadows from the distance. For more depth, use at least medium. "high" increases the quality once shadows are drawn (with "medium").
Screen Space Ambient Occlusion: you definitely want that NOT disabled. It adds a lot of depth. Ultra settings noticeable more than high in some indoor areas (shelves, etc.) at only a slight performance hit.
Reflection and Mirror on high. There aren't THAT many reflecting objects in RDR compared to let's say Control. So the instances where they appear you might want them good looking. Performance hit isn't too big.
Particle quality is the only setting requiring the game to restart, hence it makes comparisons hard. I didn't see no difference but expect a slight performance boost, hence it's on medium. Snow storms etc. are very taxing, so this might help for that.
Tessellation: higher settings add more complexity, while ultra adds only minor detail only visible on side by side screenshots. For a very small performance boost I leave it at high.
TAA: It's awesome. Don't bother with FXAA or MSAA. TAA practically removes all jaggies and shimmering, if not, try the higher setting. TAA sharpening counters the blurryness (under advanced options).

I chose Vulkan because it was MUCH faster than DX12 in Doom. I didn't see much of a difference in RDR2, though. It introduces that camera stuttering bug for me, though, when playing with keyboard and mouse. So you might want to use DX12 or a gamepad when encountering that issue with Vulkan.

Near volumetric resolution I recommend medium, because low get's very blocky and "console-like". Maybe you want even "high". It does look great on high or ultra, performance hit is quite big though in instances with lot's of volumetric light shafts.
Far volumetric resolution's impact isn't that high, so is the visual impact as well. I leave it on low for a small gain in performance.
Particle Lighting quality: Didn't notice any difference so I left it on high.
Soft shadows: differences are very subtle and only affects certain shadows by the sun. Self-shadow from Arthur doesn't seem to be affected by the sun, strangely. Left it on medium to expect a small performance gain.
Grass shadow: same as soft shadows. Differences are barely visible. Even on low, Grass and small stones cast shadows.
Long shadows: you might want them because they really look great in sunset and sunrise. It's the same as in GTAV and long shadows were needed when a car's headlights should cast shadows. Instances in RDR2 are obviously more rare, but I'd leave them on.
Water reflection: you definitely want that on high. Medium and low are very blocky, blurry and have visibly less detail. Performance hit isn't that huge.
Water refraction: couldn't test it yet, left it on high.
Water physics quality: you want that as low as possible. Higher settings are literally KILLING the performance for barely noticeable effect. In fact, I didn't notice anything game changing. So use only a few clicks, maybe even go for the lowest. Water still looks awesome.
TAA sharpening: personal preference.
Reflection MSAA: I expect a performance hit while I don't think that the majority of reflections need MSAA. Didn't test it thoroughly, though.
Geometry level of detail: another huge performance slider. Don't use the lowest setting as it flattens many objects in the distance and removes entire geometries at distances. The second lowest enables real geometry in the distance. Further levels only increase accuracy of said geometry. Differences only noticeable on side by side screenshots.
Grass level of detail: increases draw distance. Huge performance hit. If you aim for 30fps, increase for excellent graphics fidelity, otherwise use lower settings but again not the lowest.
Tree quality: Affects very far draw distance. Performance impact from low to ultra is about 5%. Difference only visible on side by side screenshots, though.
Parallax Occlusion Mapping: I want as much details as possible on textures so I chose high. Performance hit is very small but measurable.
Decal quality: shouldn't really have a huge impact. Just in case ultra is bugged or too much when blood and bullets flying around I left it on high. It should only affect performance when taking a lot of space in your screen frame.
Fur quality: I love fur. Gimme the best fur available! It should only affect performance when taking a lot of space in your screen frame.