Thanks Thomas.
So to recap for the OP, Polar Pro are probably the leaders / most popular ND or ND/PL filter supplier for drones including the M1P.
I'd agree with this, since my NISI set supplier now no longer makes them for the M1P
There are certainly other brands of filter manufacturers that are good, and make filters using multi coated optically coated glass . . . just watch weight, the M1P gimbal likes 1.0g to 1.2g, any more and you might get gimbal start up calibration issues and / or gimbal overload.
It's really important with the M1P gimbal to make sure filters are pushed right on down to the ring on the camera barrel.
This needs to be done with drone powered down, and preferably with the gimbal clamp on.
OP should keep in mind, benefits for video are mostly from the ND side of the coatings, these helps bring shutter speed to roughly double frame rate for nice motion blur to the eye.
PL can help with reflections (water, ice etc), and getting some colour 'pop', blue sky etc, but then so too can NC-UV filters.
Using PL side for reflections is understandably problematic for use on drones, where you need to rotate the PL to suit light and direction of light to reduce reflection on water etc, but . . .
If you set up to do a special run, then flying different directions won't interfere with recording, just that you won't get the full PL benefit unless rotating to suit the new direction.