> Maybe it would be wise to use Venti on that unsed space... ;)

If you don't have a venti server somewhere near your pi yet, then
definitely yes.

It will be a fight... ;)

I just created a new partition of PLAN9 type there (according to https://9p.io/wiki/plan9/setting_up_Venti/index.html), saved and rebooted...

...and system failed to boot:

boot: can't connect to file server: '/boot/kfs' does not exists

Pavel