I would _love_ to see more DIY mouse options. I feel like the mechanical keyboard crowd has so many options.

I've been dreaming of a set of lego-style bits of a mouse that can be assembled together... want another button? here you go. Want it on the side? Modify the 3D print file. Want bluetooth? Use this board... Want USB-C? Use that board... Want both? We've got you covered... Want a hyper-scroll wheel? Well, Logitech has a patent on that one, but here's the closest thing you can get on a DIY mouse. Now click these buttons in the configurator and hit "upload", and the firmware is installed to use your new mouse on any machine.

On the subject of adding more buttons, I think there needs to be a rethinking of mouse button events at the OS level. Gaming mice with 12-20+ buttons have to resort to creating keyboard events with weird key combinations because there aren't actually that many mouse events, which is insane. There are currently only 12 valid integers (12 types of "click") sent from the raw mouse events. Those need special handling because the numbers are chosen very strangely, but why can't we agree that for any number within some range, the odd number is a key-press and the even number is the key-release, or something like that? You don't have to create named events for all of them, but the raw integers should be valid even if you have to use the lower level events.

If I want to build a mouse with 32,000 buttons, the limit should not be the operating system's mouse event.

There's a YT channel called optimum and he made his perfect mouse and brought it up to a product stage. It may give you some ideas (like the sensor PCB is a set you can buy). https://youtu.be/oMUEsz71_xQ

Totally agree. Mouse preference is just as personal and maybe even more subjective than keyboard preference is, and even with the plethora of commercially available models, someone is going to be left settling because their needs aren’t quite met.