Shameless plug of my failed project: https://paulrouget.com/k8mod

The idea was to literally fit a Nordic SoC in a Casio watch, and update the screen to show more segments.

I never managed to find someone to manufacture the screen sadly.

I remember reading about another Casio retrofit:

https://github.com/carrotIndustries/pluto

That version did not have wireless comms - they touch on that in the faq.