I went with this in the end as it is recommended by a i3 maintainer in a Reddit post.

You have to install i3ipc in your global Python for this first (pip3 install i3ipc). Then add a startup script that runs:

/usr/bin/python3 ~/.config/i3/focus-last.py And add this to i3/config:

bindsym $mod+Tab exec "/usr/bin/python3 ~/.config/i3/focus-last.py --switch"