Since the left mouse button on my mouse is barely operational, it's time to replace the microswitch. I opened up the mouse, and it looks so simple even I can do it!
The interesting part is that the mouse PCB has a provision to add a third mouse button. I had read that the 2-button A3000 pregnant mice were so designed (seems obvious since Commodore shipped a 3-button variant with the 3000UX), but I didn't know other Amiga mice had the same design feature. I'll probably add the third switch and figure out how to make a button out of it later.
The mouse itself is the one shipped with a lot of 600s, 1200s, and 4000s - arch-shaped with equal-sized buttons.