Target Gamut: I did try 2020 and it gave everything an olive cast so went back to Auto (709). As tij suggested, DCI-P3 is probably the widest colour space that realistically can be supported anyway on consumer devices (as I see that UHD BD are typically mastered in 2020 but limited to DCI-P3 for this reason).
The problem here isn't the color space to use, but what the TV expects. On my LG OLED I have an option to swap between BT.709 or BT.2020 for SDR signals (HDR is always BT.2020, I believe). The output of the renderer has to match that, there is no other options. So if I swap it in the TV to BT.2020, then the desktop gets super oversatured - as expected, but I presume JRVR would look just right if its in BT.2020 output mode (I didn't test that yet, but theory holds).
If you are using Windows HDR, DWM probably expects SDR BT.709/sRGB and converts to HDR BT.2020 for output - hence it expects JRVR to output BT.709/sRGB, like any normal desktop application would, but if you send BT.2020 instead then you get the typical under-saturated look with a slightly green hint - just as if your TV was expecting BT.709 and you send BT.2020.
Presumably Windows can be informed of the gamut you are sending, but thats a task for another day.
I don't know if any TVs can be set to expect DCI-P3. Maybe some slightly older models had that before BT.2020 properly established itself?
The only reason not to use BT.2020, if your TV can accept it, would be to avoid banding in 8-bit signals, due to the much wider colorspace and the same amount of bits. JRVR can't output 10-bit yet.
Its important to remember that both the Gamut and Gamma options are not about the source image, or your preference, its entirely only about what the display expects to be receiving (or if you use Windows HDR, what DWM expects) - and displays will expect BT.709 or BT.2020 - even if they can only display DCI-P3. Unless someone is going to tell me that you can also set your display to expect DCI-P3, then I can add it.
(Gamma can be a bit about preference, to be fair.)