Plex for audiobooks. It’s amazing. There are android and iOS apps too. It will keep your progress across multiple devices just like audible.

I use an N5105 for my Opnsense firewall and run Adguard Home on it as well. Installing software on it was incredibly easy. Mine was barebones, had no OS, and it has an American Megatrends BIOS so it was as easy as selecting boot from USB.

I have no doubt it could run a small homelab. I have my Unraid running on a worse quad core gen 4 i5 processor (20+ docker images, 1 VM).

The only downside I would point out is that some of these things are passively cooled and put out some HEAT. It looks like yours has a fan so that’s nice. Maybe another would be Plex transcoding… depending on how many people you have on your Plex you maybe could run into some transcoding issues.