For number 4, consider switching to e.g. KDE which is an alternative desktop environment you can install in Debian.
If you reinstall, consider Kubuntu, which is Ubuntu but with the KDE desktop. Search for screenshots first so you know if it is somwthing you like.
Number 2 is by design. Running as root is extremely dangerous, and passwordless sudo is not much better. You can, of course, allow sudo without a password by editing the /etc/sudoers file, but be concious of the security implications (any program you run would essentially have full access to everything, without you ever knowing).
Discord has a weird and confusing definition of “server”. The equivalent in Matrix would be “spaces” but they are not very commonly used (and I’m not sure there is a public list). Instead Matrix is most often used with individual rooms.
Due to the distributed nature of Matrix it is actually impossible to create a complete list of public rooms. However, one probably fairly complete list can be found at https://view.matrix.org/. Most clients have room search built-in, so you would rarely need that list
Isn’t OIDC basically what you want? You just need to convince the forums to use it.