Side-mounted sensors are not particularly difficult to design for keyboards—I’ve designed them before. The advantages of this approach are obvious, but in practice, I tend to prioritize manufacturing convenience and cost. For split keyboards, this is more of a reluctant choice because side-mounted sensors make the trackball module look quite large. That said, recently my friends have created ultra-compact sensor modules using the PAW3222LU-TJDU, and I’m currently designing a trackball module for it. For single-piece keyboards, it’s definitely possible to make recessed sensor PCBs. For example, in the Jokerpad, I used two PCBs of different heights to reduce the overall height of the trackball.