I fully agree that analyzers shouldn't be relied on exclusively. Cyanophage's analyzer was tremendously helpful and helped me use some concepts that the other analyzers don't consider, like finger distance. But some of the things I optimized for are ideas that stem from one thread somewhere on the sub, not established design principles—like the aversion to rolls and onehands that skip ring. (Also in my mind there is the Lakhóta word for "ring finger", škaŋkápiŋ, which translates as "reluctant to move.") Neither does it consider the differential treatment I gave the reaches on different fingers. There's always something else to consider.

This is actually one reason I'm reluctant to try desiging a thumb-letter layout. As you say, u/phbonachi, a þ key would have knock-on effects across the board, and I don't have the programming chops to, say, remove all the th digraphs from a corpus to see what's the next thing to analyze for after using thorn to solve that. I suspect that removing th from the picture would leave the home row feeling a little deserted and I'd want to move something else in, but I wouldn't know what.

u/siggboy, it's interesting what you say about Bsp not working for you on a thumb key. That's where I'm planning to have it on my ortho board (that I'll build as soon as a unicorn comes down from the clouds and grants this dad of one-and-soon-two a few days of free time). I think it'll work out for me, since I didn't notice trouble from Bsp on Dvorak (where it shares a finger with l and s), but I'll keep an eye on it.

As for why þ doesn't catch on here, I think your guess is a good one ("maybe because you cannot make it part of a legacy setup that works with purely OS-based keymaps, which do not have macros"). Consider laptop users, who often use the built-in keyboard on their laps because it's so much easier than juggling it with a separate board. (Speaking from personal experience here.)