Why a separate partition at all?

I habitually move the "root" file system to /_r. (Actually if you install Debian to a btrfs it does that out of the box.) Along that I'm using an OS independent /home subvolume and a /btrfs subvolume and the encfs subvolume and … you get the idea.

This makes way the root FS is fairly clean; also I can clone the subvolume and then run a test installation, either directly or via systemd-nspawn, without the test stuff being visible from the root *or* vice versa.