The best free alternative to Astropad is Weylus, which is both free and Open Source. If that doesn't work for you, our users have ranked five alternatives to Astropad, but unfortunately only two of them is free.
The list of alternatives was last updated Jun 1, 2021
superDisplay turns your Android device into a portable USB display for your Windows 10 PC. Duplicate or extend your screen simply by plugging in your phone or tablet. View 7 alternatives to superDisplay