So the Reed Switch worked as expected, no brainer of course.
I put together some code to compute the RPMs and display them on the Seeeduino Extender OLED display. If I display the result in RPMs, the output seems to be reasonably right, but if I divide further the fluctuation is bigger. Time to debug.
Current code is here, I am suspecting something is wrong with the 5ms forced delay in the IRQ.