I use an Ergodox EZ. You can check out my configuration here. You’ll see I recently started playing around with implementing some of the kakoune like movemet/selection at the keyboard level using layers. It works fairly well, but I havent quite figured out how to do things like the I and c commands that combine a movement and modeswitch or deletion or all three. It’s also a little wierd as I can’t really map the escape key in my native layer to get “back to normal mode”… In any case you might find some concepts from it interesting.
To more directly answer your question, I wouldn’t still be using kakoune if it weren’t for the ergodox, just based on the extra strain of chording alt with everything. To counter it, in my home layer I use the dual-function capability to make z and / ctrl when held, and x and . alt when held. Moving the fingers ortholinearly down one row and holding is FAR less strenuous, especially since its so symmetric and can be used from the opposite hand to the key you’re modifying (<a-i> becomes hold-x with left hand, i with right, etc).
I find the ergodox hugely helpful in other ways, such as making numpad appear centered on my right hand home row when a layer is toggled to get numbers, or making the number row only send the special characters (without shift key) so reduce usage of shift in programming. But probably the biggest thing is the ortholinear arrangement (rows directly aligned rather than offset), which makes transitioning to different rows feel so much more natural. It feels like unneccessary contortionism to use a regular keyboard now.
About the thumb clusters, yeah there’s honestly too many keys there for your otherwise very clumsy thumbs. They’re pretty much laying sideways against the cluser and you’ll find its much more difficult to figure out which one you’re on by feel with the side of the finger rather than the much more sensitive fingertips. The furtheest keys are definitely a stretch. I just dont map anything important to them and instead use them for rarely used keys like print screen, or for modifier key combos (ctrl-p for my editor, and such). Its really not that big a deal from an ergo perspective, but from a “its kind of a waste” perspective it can bug you. You might want to look into Redox or one of the other offshoots that put fewer keys in the cluster. The catch being you’ll have to find a DIY kit to get one of the others (or someone who will build it for you). Take a look at this visual interactive comparison of the ergo family split keebs: https://jhelvy.shinyapps.io/splitkbcompare/