Hard to achieve tracking "sweet spot" on PMW3610 with 25mm ball

[R1AACR]

Issue Description

It is almost impossible to achieve accurate tracking using PMW3610 (labeled as "Skree (zmk)" in the configurator) with a 25mm ball and static bearing balls. The tracking issues appear as very slow movement on one or both axes and some movements are not registered by the sensor at all.

Hypothesis

I believe the issue stems from the relatively large bearing balls (3.175mm is currently the only option) for the small 25mm ball. This combination lifts the ball approximately 1mm above the sensor, causing tracking problems.
The sensor works perfectly without bearing balls, but the trackball is obviously not usable without them.

Suggestions

1. Add support for smaller bearing balls (but to be honest 3.175mm feels great with 25mm ball)
2. Adjust the sensor position to be closer to the ball in the generator

My Current Setup

• PMW3610 with this PCB
• 3.175mm ball bearings
• 25mm ball
• ZMK with this PMW3610 driver on nRF52840
• Several keyboard designs (example)

[R1AACR]

I've tested locally with reduced sensorDistance=1.6 on the same keyboard as in the example (without pro features), and it worked perfectly. The tracking now works as expected.

While this might not be the ideal way to fix it (even if we add a specific condition for 25mm ball with PMW3610 only), it's a working solution that needs to be tested with other bearing types that I don't have right now. I also considered fixing this by adding an additional sensorDistance "variant" in expert mode, but that would increase the number of pre-rendered models and build time.

So I don't know the proper way of fixing this and would like to hear the best approach from @rianadon