I'm typing on a SoflePLUS2 right now. It's based on the Sofle v2 design, which is described as having a 5-key thumb arc per side. But I try to limit thumb use to the innermost 2 or 3 keys per side after experiencing fatigue. I use the outermost 2 or 3 as opposite-hand modifiers (ctrl, opt, cmd) and try to pull my whole arm in to use them with the same-column finger, instead of treating them as a thumb key that requires folding my thumb underneath my palm as I keep my hand in the home position.

It seems like many in the ergo keyboard crowd are trying to never move their hands from the home position, and I think that might be a mistake. Use a variety of muscles, avoid unnatural positions. More broadly, my understanding is that the research behind using a tented/splayed split keyboard is solid (better shoulder through wrist positioning) but there's nothing really but anecdotal experience supporting the idea that vastly reduced key counts (and associated need for complex layer setups) or column-staggered layouts reduce pain and plenty of confounders (going from unibody to split simultaneously, maybe switching from QWERTY at the same time too, reducing speed, often learning decent form for the first time, often regression to the mean because people switch when they are having problems).

My previous keyboard was a split with traditional row stagger (Goldtouch) that Google's ergo team advised me to try forever ago. I switched recently because I wasn't liking the mushy feel of the keys, that the two "space bars" weren't distinguishable, that it doesn't have an integrated pointing device, and that after such long use I'd worn down the homing indicator on the f/j keys and was struggling to orient my hands correctly. But row-staggered layout was fine IMHO. Made it easier to learn, to switch between it and other keyboards when I had to, and to hit keys further from the home position.

Here's something from Kinesis, who have been designing split ergonomic keyboards for a long time: https://kinesis-ergo.com/wp-content/uploads/Advantage360-ZMK... search for "If your thumbs are sensitive" and "Guidelines for using your thumbs". And note that while they have keys under z/x/c they do not describe them as thumb keys.