If opkg runs out of space, it will not remove the files it was installing. Run this custom script in order recover the space by removing these files and the installed dependencies.

opkgclean.sh

#!/bin/sh
#takes one argument/parameter: the name of the package which didn't install correctly and should be removed along with its dependencies
#do opkg update first
#example: ./opkgremovepartlyinstalledpackage.sh pulseaudio-daemon

#get list of all packages that would be installed along with package x
opkg update
PACKAGES=`opkg --force-space --noaction install $1 | grep http | cut -f 2 -d ' ' | sed 's/.$//'`
for i in $PACKAGES
do
        LIST=`wget -qO- $i | tar -Oxz ./data.tar.gz | tar -tz | sort -r | sed 's/^./\/overlay/'`
        for f in $LIST
        do
                if [ -f $f ]
                then
                        echo "Removing file $f"
                        rm -f $f
                fi
                if [ -d $f ]
                then
                        echo "Try to remove directory $f (will only work on empty directories)"
                        rmdir $f
                fi
        done
done
echo "You may need to reboot for the free space to become visible"

Worked perfectly with OpenWrt 21.02.1 and TP-Link TL-MR3420 v5. Thanks, you are my saviors!