One important detail: MSI afterburner reports under 50% CPU usage (on all threads) and 100% GPU usage while the FPS is around 20. That is WAYYYY lower than the benchmarks suggest.
This is likely because you have triple-buffering with vsync enabled. Vsync prevents screen tearing by having the system wait until the monitor's next refresh cycle before sending the completed image to the screen. Triple-buffering tells the system to keep rendering new frames even if they are not being drawn yet. So, depending on which metric is being looked at, different numbers can get reported. Generally, vsync isn't going to be great unless you are maintaining frame rates higher than your screen's refresh rate, which appears to be 60Hz in this case. What monitor is it? Does it happen to support G-sync/FreeSync?
RDR2 is really just a very demanding game, and is currently one of the most demanding on graphics hardware if you max its settings, going beyond what is reasonable for today's hardware. And some of those settings can cause big, double-digit hits to performance when set to ultra, in some cases without significantly improving visuals.
For example, having water physics maxed is going to tank your frame rate around bodies of water, so you probably don't want the slider all the way to the right, and it should be lowered to around the 50-75% range. However, water reflection quality causes only a minor hit to performance, so you could actually turn that up on high. Water refractions is somewhere in-between, but medium can improve performance a noticeable amount, again, without having a major impact on visuals. There are also a number of other settings that can negatively affect visuals, but don't really improve performance much, and should probably be left on ultra.
Hardware Unboxed did a good pair of videos when the game launched covering the relative performance hit and impact on visuals for each setting, so they might be worth a watch. You don't necessarily need to follow their recommendations entirely though, as some of the settings they suggest lowering only amount to around a 1-2% performance gain over the next highest setting, which is probably not going to be too important for a high-end graphics card like a 2070 SUPER running at 1080p. Others however, like water physics (when near water), lighting quality (at night), reflection quality, and MSAA, can cause very large hits to performance, potentially in the 20-40% range. And a lot of other adjustments can improve performance by around 5% or so, often without noticeably affecting visuals, so some of those might be worth adjusting too.
Their first video covers the basic settings, which will tend to have the largest impact on performance, while the second video covers the advanced settings, which are mostly linked to the basic ones, and can be used to extract a bit more performance or a bit better visuals out of them. Those water quality settings I mentioned before are detailed in there...
View: https://www.youtube.com/watch?v=385eG1IEZMU
View: https://www.youtube.com/watch?v=C3xQ33Cq4CE