It was something like
apt install task-mate-desktop
(Look it up, I may remember wrong)
that installs MATE. Do this BEFORE removing Cinnamon.
After that removed Cinnamon
apt purge cinnamon*
apt autoremove --purge
I should check these commands again on a VM, to see wether they really work as I remember they worked, just don’t have the time for it today.
Tomorrow I can do it if you whish.