When I see this much effort, it just makes me think that, at some point, maybe it's worth ditching the whole concept of giant buttons: https://www.charachorder.com/products/charachorder-one

I'm blanking on the name, but there was an older 5-way switch keyboard, where the 4 non-thumb fingers went in a recess with 4 paddles and a push-button; I've known several people who used that because of severe RSI, but it was quite expensive.

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Searching split keyboards on google image search for long enough found it: https://en.wikipedia.org/wiki/DataHand#/media/File:DataHand_...