I have a laptop with only 11 inches display.

I'd like to get some tips/plugins for maximizing screen space.

I've already set every panel to autohide.

In gnome shell I use the pixel-saver extension to merge activity bar with title bar.

Is there any plugin, patch, workaround, or advise, to achieve something like that in kde neon?

Maybe it's too late, but you can add "Active Window Controls" and "Global Menu" to you top panel.

I understand it's too late, but it may help for people who come later.

Merge Title bar buttons Global Menu : https://github.com/psifidotos/applet-window-buttons

Merge Application title : https://github.com/psifidotos/applet-window-title

These both are working fine for me.