For serious ergonomics, splits are the way to go.
I don't recommend the Ergodox EZ just as you note the thumb cluster is pretty terrible in actuality.
Is my recommended 'out of the box' keyboard.
As you also rightly point out, there's a learning curve with a move to these kinds of keyboards but since you're on an ergodox, the gap should be significantly smaller.
My daily driver is a Diverge 3 (company no longer operating), but the best one so far. Close in, thumb clusters with few, easily accessible keys. The thumb can really only access 3-4 keys easily.
The real one I want to go after is the Dactyl Manuform... but omg there's no nice way to get at that thing. And the cost is so high just to 'try it out'.