I highly recommend RSTHD layout.
The optimizing makes it really comfortable to type with, and only a few common words are a little awkward to type.
@Anarkat @thegibson @benbrown I really like the idea of moving letters onto the thumbs. This hadn't occurred to me because I was still trying to use lettered keycaps, and the thumb keys are a different profile ...
I use blank keycaps. You can't really see the key faces when the split boards are in a good ergonomic position, and all of your commonly-used keys should be on layer-1 anyway, meaning that you'll have them memorized fairly readily.
To me, the purpose of keycap legends is for making the KB usable to people who aren't familiar with it.
In the case of a customized personalized device like a split keyboard... why would anyone else use it?
It's like sharing earplugs. Get your own lol
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RSTHD is a layout designed/intended for split KBs with thumb clusters
I modified mine by switching D and P to make it into RSTHP.