I have a pretty clean signal from the crankshaft sensor (variable reluctance i believe) that looks like that in the attached scopes pictures, and it is generated by the wheel shown in the last picture, for reference.
I thought to scale it down first with a voltage divider to get the flat areas below the low logic level, then trim it with a 3.3V zener, but I'm open to better suggestions. A square wave would be nice to have but i suppose also not strictly necessary. I also have some optocouplers, schmitt triggers and 555 laying around as well.

As for the measurement, I suppose I can measure the largest delay between any given rising edge, which corresponds basically to diametrically opposite teeth, multiply by two and invert the result to obtain the frequency in Hz. But i will think about this later once i get a suitable signal to play with.

at low rpm

at high rpm

https://preview.redd.it/7esfxcg3zecd1.png?width=403&format=png&auto=webp&s=77683577dd5fcc73d120883b4cfaceacb46786a3