A Homebrewers’ “How-To” FAQ*

Just to save y'all the trouble, you cannot have the homebrew subspecies be nested under the homebrew species the way official stuff are.

You can make a subspecies of an official race and do it but not for homebrew species. That has never been a thing and is unlikely to become a thing due to D&D 5.5e moving away from subspecies.

Your best alternative is using the Options system to set the subspecies as Options instead of being set as subspecies - this is the way new 5.5e does them now, such as 5.5e version of Elf or Tiefling.