Building the mouse Logitech won't make

I miss the old microsoft track ball. The ball wasn't a thumb exercise as it was under your index-middle-ring fingers and you clicked with the thumb.

Don’t current Kensington trackballs have this layout as well?

I've been using a Kensington Expert Mouse Trackball for several years now. You might like it for a center trackball position. It also has a ring around the ball as a scroll wheel and four buttons which are easily programmable. A couple of AA batteries last me about a year on the 2.4GHz wireless, many months on Bluetooth. The ball comes out really easy so it's easy to take apart and clean. It has handled almost a decade of near daily use without any parts breaking.

https://www.kensington.com/p/products/electronic-control-sol...

Not quite, with the thumb buttons like the Trackball Explorer had. Closest I can find is the DEFT Pro https://elecomusa.com/products/deft-pro-trackball

They do, but I've had severe quality/lifetime issues with them. Three Kensington trackballs, two different models, all three lasted under a year for me.

Strange. I have lots of Kensington Expert Mouse trackballs, some of which I have used for 7+ years, and none have ever given out on me. I have a stockpile of spares in case they stop making them, but they seem to be built very well.

Which models did you have problems with, and what went wrong?

Never tried the Expert Mouse in particular. Two of mine were slimblades, one orbit with scroll wheel. Different failures each time -

Orbit - broken plastic around one of the bearings. ball no longer turned smoothly or straight in every direction, and dragged. Slimblade #1 - began to operate erratically. I believe it was a failure of some kind with one of the optical sensors, but I never was able to figure it out consistently. Slimblade #2 - microswitch under the LMB failed mechanically, no longer triggered.

the slimblade's were provided by my employer at the time, the orbit was purchased personally for at home - it's use overlapped with the slimblades at work.

I don't think I'm super unusually hard on my trackballs - My Elecom Huge lasted for ~6 years before the soft touch plastic finally got a bit gross from skin oil contact, but still was functionally fine, and my current protoarc is going strong two years in.