I'm curious why none of these split designs ever feature a trackball on the right half integrated into the board somehow. There are plenty of FOSS trackball designs, it seems like it would just make sense, especially for ergonomic designs, to have a trackball set up where you don't have to move your hand over to move a cursor. Also I don't understand why they're always missing function keys.
The Ultimate Hacking Keyboard supports a trackball or trackpad attached to the right half. The expansion modules just recently became available though so it may be hard to find reviews or people with hands on experience.
Edit: are you sure they started shipping? the shop page still says under development?