I don't think it is possible to hide specific hotbars within the crosshotbar itself. The only one I know you can hide is the 8th but only as the additional display (WXHB). "/chotbar display" seems to be invalid alltogether. I have set up the WXHB one as kind of "menu" with macros that will change the displayed hotbar, which I then treat as "submenu". Hiding normal hotbars by number works fine, the crosshotbar doesn't seem to allow that tho. I am not sure if that is intended behaviour, given it is listed as valid option.