I believe that the target primaries must be those of your monitor.
BT.2020 are the primaries used to encode the HDR movie.
Monitors with such primaries are rare and extremely expensive. For most good monitors "--target-prim=dci-p3" should be appropriate, if DCI-P3 has already been selected from the on-screen monitor configuration menu, because most monitors come configured by default to use the ugly sRGB color space, so you must change the defaults to be able to display content with bigger color spaces.
Some monitors might have the option to use the BT.2020 color space, when they would still not be able to display all of it, but they would do internally the conversion between the BT.2020 used to encode the movie and whatever color space the monitor can actually display.
BT.2020 are the primaries used to encode the HDR movie.
Monitors with such primaries are rare and extremely expensive. For most good monitors "--target-prim=dci-p3" should be appropriate, if DCI-P3 has already been selected from the on-screen monitor configuration menu, because most monitors come configured by default to use the ugly sRGB color space, so you must change the defaults to be able to display content with bigger color spaces.
Some monitors might have the option to use the BT.2020 color space, when they would still not be able to display all of it, but they would do internally the conversion between the BT.2020 used to encode the movie and whatever color space the monitor can actually display.