I know this thread is about custom mouse creation... but you did ask about a better mouse... so ...
I use this.
because
That's not my hand. It's just a visual reference.
My middle finger tends to use the scroll wheel, the two finger adjacent to my middle fingers use the the top two buttons and the the bottom two is delegated to my thumb and pinky.
It comes with this rest thing but I threw it away because I find it useless on my desk.
as for DPI, the following chart list some stats on the best different models of trackballs.
https://i.imgur.com/0308O.png
came from this post on reddit
this is one interesting thread concerning the subject.
I have been using this mouse for the last ten years.
I have played a variety of games (FPS, RPGs, etc) over the years so it works quite well for my taste.
I think the only reason the manufacturers don't release the DPI or the polling rate for trackballs is because that's not that important to them. The important bit is the accuracy of the sensors.