How to package a PostgreSQL extension as a binary .deb file in Ubuntu?
Is there a way to combine the manual approach above and the PGXS
  approach to automatically create a PostgreSQL extension as a .deb
  file?


Sure. First you need to create a debian/ directory with a control and rules files copied from other extensions. There are many examples in the PGDG Apt repository. Add the following line in your APT sources (bionic is the codename for Ubuntu 18.04) if you don't have it already:

deb-src http://apt.postgresql.org/pub/repos/apt/ bionic-pgdg main


To see the list of packages: apt search 'postgresql-*'

To download the sources: apt source name-of-package

Once your debian directory is good and the dependencies are installed, basically you build the package for the extension with dpkg-buildpackage -rfakeroot -b like any other Debian package.