How to remove all packages with DNF on which no other package depends on

With Yum, you would use the package-cleanup command from yum-utils. But, with DNF, it's built in as the dnf autoremove command. From the docs:

dnf [options] autoremove

Removes all “leaf” packages from the system that were originally installed as dependencies of user-installed packages but which are no longer required by any such package.

Note that you can also use the dnf list command to see which packages would be removed, before running the automremove itself, with

dnf list autoremove

Also note that DNF has another difference in behavior which should help you out in similar situations (but not your exact case). If your main piece of software were installed by DNF (rather than outside of DNF by hand), and you used dnf remove, the default (unlike yum) is to also remove any dependencies which were added just for that package and which aren't used for anything else.