The Expert Mouse costs a lot, lot more than a simple trackball. And my subjective feeling is that it's actually less ergonomic than the cheaper Logitech I've got.

If you’re open to spending about the same (about $10 more), then the Logitech MX Ergo might be exactly what you’re after.

https://www.logitech.com/en-us/products/mice/mx-ergo-wireles...

I cannot use that kind of trackball. It relies on actuation with my thumbs, and it hasn't worked out very well in the ones I tried. I prefer "ambidextrous" ones with the ball in the middle.

Well for the record then this is my jam and I cannot express how much I love it: https://www.rtings.com/mouse/reviews/elecom/huge-m-ht1drbk

There’s also the slightly smaller Deft Pro (https://switchandclick.com/elecom-deft-pro-review/) which I also own and use for travel — it has a superior scroll wheel and therefore may be a superior choice for you.

For me the Huge has a perfect form factor that outweighs all else.

On mac I use steermouse to map the buttons.

I promise to stop being a trackball pusher now.

That Elecom is reasonably priced. Thank you for bringing this to my attention.

Maybe GameBall[1]. It have 6 buttons and vertical and horizontal scroll.

[1]: https://www.gamingtrackball.com/