Hi! Is there a way to change the default units from 'px' to 'mm'? Similar to 'File/Document Properties.../Units' in Inkscape? Thx!

Krita doesn't get involved with printing and so images are measured in pixels.
There is a convenient conversion to distance process that can be seen in the Measure Tool options in the Tool Options docker which is a simple conversion based on the document ppi value that is entered when a document is first created and is stored in the document properties. This is built in conversion calculator for user convenience. Printing and all its complications are totally up to the user.