Haven't used Debian in a while now, but back when I did, Wayland never did appear in the sessions dropdown on a fresh install with an nvidia card and nvidia proprietary drivers. Doing what is explained in the following link always worked for me though:

https://wiki.debian.org/NvidiaGraphicsDrivers#Wayland

Apologies if you have already tried this, but as I said, I've luckily not had to do more to get it to work, so hopefully it's the same for you.