By following the EXTROOT configuration guide, I was told to remove the packages I have installed:
These devices should have enough space to install the packages we need. Remove all packages you have installed to add functionality, as they are only wasting space now. After you make the extroot you will have all space you need.
But, how to do that?
opkg list-installed isn't what I need, because it lists all installed packages, not just the ones I have installed. Neither is
opkg list-installed | cut -d- -f1 | opkg status | grep -B 3 "user installed", because it still lists more "user installed" packages than I really installed.