Just orderded a Freewell ND1000 saw other day below picture taken never thought 10stops ND also available.

I think you can see the problem with an ND1000 right there.
The Phantom will not be totally stable for the length of time the shutter is open and you will get motion blur.  You can use these very dark filters to do as this chap has done, i.e. smooth the sea, but you have to have your camera perfectly still to get the intended result.
IMHO you will never get a sharp shot with this extreme level of attenuation.