I can’t say anything re tool speed but I can say that you can’t mount a pen rigidly. The pen needs to be spring loaded and you set the zero point just a tad below the paper so that the pen can press against the paper but not get forced into the paper (hence spring pressure)
The subject has come up in this forum (ie Spring loaded pencil holder for Altmill and others). You could also look at Thingyverse and search for ‘mill pen holder’ or similar. Even if you don’t have a 3D printer, it will give you ideas.