I think GNU Guix System scratches all my itches:

• Committed to being 100% free software even at the kernel level (I know this is controversial)
• Focus on reproducible builds
• Atomic updates that can be rolled back if something breaks
• A package manager that makes it relatively easy to package software (there are importer commands that can import from language-specific package managers such as pip and cargo) and makes it possible, as a user, to apply transforms to packages (i.e. build with X commit or with Y patch)
• Per-user profiles (in addition to the root profile and the system profile) allowing user to install software without requiring root. Users can even create separate profiles as well as throwaway profiles for running scripts or one-off commands (i.e. a python or bash script can use guix shell as its interpreter listing all the packages it requires).

Previously I used Ubuntu from 2008 to 2009, Trisquel from 2009 to 2014, and Debian from 2014 to 2019.