I have automatic redirect (with the LibRedirect-plugin in Firefox) of any YouTube-links so that they open in Freetube. Set up on my phone as well, and it works nicely.
I can't actually find the View-menu (or any other of the usual File, Edit etc.). Maybe it doesn't play nice on KDE under Wayland? In LibreOffice Impress I can right click the gutter and click "More" and bring up what is called "Options - LibreOffice". Here I can find "Peronsalization" which allows me to select different themes (to no effect) and that's it.
From my experience, OnlyOffice provides better compatability with MS Office-files (that is, more so than LibreOffice). However, having used Powerpoint quite a lot in my professional life, and using OnlyOffice Presentation to make a slide deck now, that is an area where I unfortunately find it severely lacking. There's also the issue about their license - I am not all that familiar with it, but apparently they are not as free and open as they claim to be.
For installing plugins, I am fine with it, but would not want any telemetry being sent somewhere without my knowledge. The data collected should stay on my server.
I have Proton VPN running on two different machines running Ubuntu-based distros with KDE and Cinnamon, respectively. Works fine :)