If you consider ergo keyboard with thumb cluster, almost all of them would suffice.
However, like you, I want a standard keyboard with split spacebar.
I don't want to adapt to ortholinear layout just for having access to thumb cluster, AND giving up inverted arrow cluster and page up/down home/end.
To answer your question, kbdfans DZ60 REV 3.0 60% solder pcb support 3 way split like so
KBD67 rev2 65% solder pcb
kbdfans 75% pcb has the 2.75 1.25 2.25 fixed in one orientation (which fortunately favors the right thumb getting to 2 keys)
they all require solder, and I don't have the time so I bought a complete assembled kbd75 keyboard from them and loving it so far.
Having alt, shift, space, ctrl all within thumb access and enabling one-shot modifiers/sticky key, and I'm in emacs heaven. (and also don't mind playing typing test on typeracer with Capitalized Paragraphs)
ymdk wings and other alice styled keyboards also have split spacebar, but the right thumb can only comfortably access one, and i want both thumbs to have access to two keys, so I personally ruled them out.
also there are split staggered keyboard like this
but this particular one suffers the same problem as alice style keyboard mentioned above.
ikki68
https://shop.wuquestudio.com/products/ikki68-aurora is another keyboard that is hotswap and supports yet a different split
but it's only available through group buy. The split is still unfavorable for right thumb accessing two keys, but it's slightly better than the alice style.
finally there's another company similar to kbdfans at ymdkey.com that has a 60% minila pcb that supports this symmetric split
I ordered one and it should arrive in 3 weeks, between this and the kbd75, both allow my left thumb to have access to "alt" + the first split (shift), and right thumb access to the last 2 splits (space, control).
I just wish it's not a 60% but a TKL, with this minila split.
Better yet, take all the space from the existing 1.25u alt + 6.25u space + 1.25u alt, and split them into
2u, 2u, 2u, 2.75u
or
2u, 2u, 2.25u, 2.5u
that would be perfect, as the traditional left alt 1.25u is barely reachable comfortably, extending it to 2u would be ideal, and the right alt is not accessible at all at 1.25u, so having that at 2.75u or 2.5u should fix it, while having the other split at a relatively easy to reach 2u.
I might be one of the fews that want a regular TKL with split space bar (aka thumb cluster).
If I am knowledgeable or resourceful enough, I'd run a group buy for such a design.
