Am I right in assuming that Virtual Desktops are KDE’s equivalent of workspaces in i3? If yes, do give i3 (or one of it’s derivatives) a try.

Arch by definition is bare bones, you install what you want or need. Because of that, the installation process can be daunting for newcomers (hint: use the Install guide on their wiki), but you get full control.

I used Arch and Fedora with i3 on my old laptop, imo battery life with those was good.