I use KDE. Some use GNOME. Most other options are probably to be left out as X11 is unsafe.
Cosmic is not nearly finished, but will probably be a bit safer, as its in rust, even though not tested.
Then there are window managers like Sway, Hyprland, waymonad, wayfire, etc.
RaspberryPi also has their own Wayland Desktop.
Is every Wayland Desktop / WM equally safe, what are other variables here like language, features, control over permissions, etc?
This is not a very good question. If you are concerned about security you need to think about what specifically you are trying to keep safe? Here are some examples of different security scenarios:
Each one of those questions has different means of securing the computer. With question 1, it is not so much a matter of desktop environment, rather it has more to do with using full-disk encryption, setting a boot password in UEFI, and always having your lock screen enabled.
With question 2, this is a much more difficult task and you would probably be better off running apps in a VM, or carefully crafting your “Security Enhanced” Linux profile – or not using Linux at all, but using FreeBSD which allows you to run apps in jails.
With question 3, be more careful with filesystem permissions and access control lists, setup your sudoers file properly, and use a desktop environment with better security auditing like Gnome or KDE Plasma.
Never heard of these jails, like bubblejail? Its available on Linux too.
I know the question is vague and highly dependend on Threat model etc. Pre-enabled services, distribution adding stuff to it, SELinux confined user (not working with Plasma at all), xwayland support for keylogging chosen keys (Plasma).
Also GTK is widely used for rust apps, this doesnt exist on Plasma at all, not a problem though as Plasma is not Gnome and simply supports GTK normally.
I don’t think the DE itself matters, but I can recommend using an immutable OS (makes it harder to install malware) and installing flatpak apps only. You can also use software like flatseal to further lock down permissions