I've also never heard of a split layout in more than two pieces. Reddit says it's been done (https://old.reddit.com/r/ErgoMechKeyboards/comments/13dkk8g), but it certainly doesn't seem like a common thing.

I can get by without the small keys. I could move backslash and caret to letter keys, set up Fn+number pad for my function keys, and do something similar for caps lock if i don't omit it entirely. I'm a little surprised there's not more demand for small keys. I guess people prefer layers/chords or some other kind of button.

My current layout for an ANSI board has my fingers on the top 3 rows and my thumb on the bottom two row with symbols and space/modifiers, where QWERTY has ZXCV... and its thumb row. My thumbs already do this so i figured it would be fine as long as the thumb buttons aren't too high up. I'll watch out for that when i make a physical model. As for the stagger, i based it on how my hands rest when they're not at a keyboard. A bigger stagger sounds uncomfortable to me but i guess i won't know until i've tried more different keyboards.

Backslashes are pretty rare for me, especially because i don't use Windows. I just put that there because it's on shift-slash on my current layout. I'll move them if i need the space for something else, but i don't see it coming up.