It certainly isn’t a permission issue like others are claiming at least for the window rules. It could be a bug, but as far as I know the steam deck does not use the latest version of kde, so it may already be fixed and isn’t worth reporting.

As for onlyoffice, if I had to guess. It is forcing its own window decorations and therefore you don’t get all of the same options as with windows that use the breeze decorations from kde. However you can still create window rules for these window. If I recall correctly. The default shortcut is alt + F3 which should work regardless of which decoration the window is using.

What does the kwin rule for evolution look like?

Lastly, the password thing is the only one that could be related to a permission issue. I however don’t believe you should start mixing keyrings. kwallet is probably already installed and running. Like I replied to one other person already. Both gnome-keyring and kwallet use the secret service standard. The only reason it could not work is if evolution hardcoded gnome keyring instead of relying more generally on secret service. However, I doubt that. You could even use keepassxc as a secret service provider if you wanted to. Although it is a bit more annoying in practice.

We’re basically already there. KDE has already moved to the secret service standard that is used by gnome keyring. The real issue in my experience has been applications hard coding a dependency on gnome-keyring and not secret service. Some applications such as mailpring have fixed that hard coded issue, but it is still an issue in other applications.

Honestly, it seems pretty similar to cockpit which should be available in most distros repos.