I was quick to feel that the 38mm ball was too tall for this keyboard. I've also mentioned that it tracks surprisingly poorly. Thus, I went looking for an alternative, and got some 35mm balls to try. 

The hope was that the lower height will be more ergonomic and the steeper angle to the bearings will reduce the chance of the ball rolling out.

Jade

The first balls I found looked black in the product image and cloudier in review images:

Conveniently, both a black and a cloudy grey ball arrived:

The black ball tracked mostly fine (and certainly better than the original grey ball) but the marble-like cloudy ball didn't at all. I correctly suspected this was because the balls were too close to the sensor, and found some m3 nuts to act as 2.3mm spacers. Now both balls would track but the cloudy ball would cause the mouse to jostle when stationary. 

The black ball felt scratchy, as if a brown keyswitch was turned into a trackball. The cloudy was smoother. Both were notably louder than the grey ball, as well as less spherical; it's round enough not to feel but enough to see it kinda wobble in place by about 1mm tops.

Stainless Steel

Seeing the below video, I decided to try a solid stainless steel ball. I got 2 since it was better value:

There's enough specularity on the steel ball for the sensor to detect features. In other words, it tracks fine. Since I got 2 balls, I was able to choose the marginally-smoother / quieter of the two.

Mass effects

The weights for all the balls are as follows:

• Original grey - 38g
• Jade balls - 62g
• Steel balls - 178g

I initially felt like 60g is the minimum and 180g is the maximum mass that I'd tolerate, though through continuous use, it does feel like my fingerprint oil makes the steel ball easier to move that the weight no longer feels like a problem. The jade ball just feels hollow in comparison. The mass of the steel ball means that I can also use it as a hand massager by rolling it around in the palm of my hand. I'd really like to try a 120g ball if it exists.

Using brass inserts as a spacer

Since the brass spacers are 4mm, I also got the idea to use M5 x 5mm brass inserts around them so that I get the 1mm difference that was lost going from 38 -> 35mm diameter. 

The balls track better, though I still feel that there's room for improvement so perhaps I need another 0.5mm of spacing.

Aesthetics

I peeled off one of the protection stickers of the frosted acrylic and now the balls look like they're in a sand zen garden. The cloudy ball has a more zen / meditation vibe and the stainless ball has a more art deco aesthetic. The latter certainly feels more Batman-esque. Here are some day / night images:

The ball is mostly illuminated by the knob LED lights, so I'll keep that in mind when considering for a better knob (as the original one is too short and is opaque).