there's an app called Sober on flathub, developed by creators of Vinegar. i use it to play roblox and i can say it works without any issues. the setup is really simple, since it dowloads everything by itself
you should try papers. it's a new app that'll replace evince as the default document viewer for gnome in a future version. it's still in development but i've been using it for a long time and it's really good.
I don't use plasma anymore but you should check in desktoo effects or kwin scripts sections of settings. If not there I'd check the keyboard shortcut list for Super + Shift to see what is being activated by the shortcut.
This is supposed to be the most used operating system, recommended for its ease of use. Meanwhile you have to sign in to use a clock app. Such a shame, especially because the focus timers are actually useful.
every chat app might use ~1GB because most of them are electron apps, which all spawn their own instance of chromium