OK, most is in the description but I did not realise how to post in any detail before.

My thoughts were.

Raspberry Pi Pico 2040  and plug in my mouse, maybe Bluetooth, unsure.
Can you plug mouse in to Pi Pico 2040? if not maybe a Raspberry Pi, I have some laying around.

Then decode the data stream modify the Y-Axis and reassemble it.

Now, maybe, pass it to another Pico which is plugged in to the PC as a HID mouse.

The PC will only see a mouse and I think this would work as well as the Ball mouse.

OK, thoughts on the hardware I mentioned above,  Yes, No, Maybe or alternatives?

I have not found, well in any form I recognise, how to capture the mouse data. (I have seen that Wireshark on Linux may be able to capture USB data, if so that is a good start to see what I am sending and then to mess with it? )
I still need to actually do it on my hardware, so do you think this possible and pointers as to software I might use?

I am aware I need to add as little latency as possible so this, if doable may be a long project optimising  code etc.

I also realise that the advice may be, NO, it is very complex, the Ball mouse is good, stick with that.

I have also thought of a variable DPI gaming mouse but they seem to have a low limit or 200DPI, I need ZERO!!

If there is a already done solution, yes of course I want it but how the heck did I miss it?

I welcome any advice as how to start and Thanks even if it is a NO.