Hi @ericmarceau and thank you so much for the lovely words of encouragement and for your added advice of fingerprint-recognition, looking into SmartCards via MacOS, the Yubico-PAM module and of course the excellent Arch Wiki regarding Yubikey. You similarly deserve a shower of kudos as well and seriously - thank you!
Ultimately, I found a workaround to this issue, which was to set up Smartcard Authentication in Gnome and then switch to the GDM Lock Screen in the MATE session.
I firstly installed Gnome and its dependencies with sudo apt install ubuntu-gnome-desktop and rebooted.
I then logged into the the MATE session, via GDM (selecting MATE instead of Gnome as the DE) upon successful boot up.
To switch directly to the GDM lock screen in the MATE session, I ran gdmflexiserver -ls in a terminal, which takes you directly to the GDM lock screen (it skips mate-screensaver) and then you can unlock the session with your SmartCard credentials there and then hop back into the same MATE session as before.
Thank you very much again to @ericmarceau , @ugnvs and @ricmarques for kindly giving your time and advice to assist here!