Eric Meurville wrote:

We are seeking a solution to monitor contactlessly (up to 10-15mm or
more if possible) the position of a small cylindrical rotor (diameter
1.6 mm, length 2 to 4mm) diametrically magnetized spinning up to 300 Hz.
The solution must not be too sensitive to misalignment of rotor and
sensor (e.g. up to +/-45°).

What's the minimum speed at which you need to monitor the position?

Can you use a pickup coil to make an AC generator and watch the phase?

Are there fast enough hall effect devices, that you could use one or
several of and interpolate position under an assumption of no abrupt
changes in speed?