I’d like to know if there is a function where I can press a couple of keys at the same time to alternate between maximized and a custom windowed size.
There are some apps that I constantly drag and drop desktop items into them (like GIMP), and such function would be helpful to me.
If I’m reading what you’re asking about correctly, I feel like the simplest way would be to set a window rule for that application to open initially with the desired dimensions, then use the maximize shortcut (Super+PgUp by default) to toggle back and forth between those dimensions and the maximized state.
Doing fancier things on demand might get into KWin scripting - KWin scripting tutorial | Developer