Leave it open and unsecured on the internet and see what happens

*arr and QBT.
push all of that to jellyfin (i'm in the process of giving uo on plex).
opensense - do a firewall on a stick.
...

Upgrade it to 9.0

Vaultwarden, home assistant, jellyfin, nextcloud.

Some more thoughts:

Some NAS stack (I use TruNAS).
Photo / image management (owncloud/nextcloud/Immich etc).
Location tracking: Dawarich / reitti.
Docker manager of some sort

Why did you set it up if you don't know what to do with it

https://helper-scripts.com/

One big VM dedicated to BOINC project of your choosing.

Tor relay ?

home assistant. openwrt. emu an XP box