I purchased a 2018 Mac Mini in May of last year.
But shiny new syndrome got me (as well as working with, and rendering, 4K footage) and picked up an M1 Mac mini which will be here this morning.
I’m looking for a use case for the older Mini. I have a pretty powerful Synology NAS so I really don’t need a server.
Any ideas?
If rendering is time-consuming and frequent, maybe as another machine to do stuff on while you wait for renders?
If you have 10 gigabit ethernet, I believe you might be able to get the Mac Mini to help with rendering on FCP and maybe other software.
I don’t know if buying adapters is worth it for you, but it’s a thought.
It could be a good automation platform - similar to Raspberry Pi but more tightly bound into the Apple ecosystem.
(If you’re that way inclined.)
I have a rather earlier Mac mini (late 2014). This runs heedlessly, and has three main tasks on my LAN:
It runs the software for my ScanSnap S1300i. Essentially I or my wife can put any document into the scanner and press the button, and it will end up as a searchable PDF in a shared folder, from where it can readily be transferred to where it is needed.
It is a print server - thus ensuring that there is a single, properly managed, print queue - avoiding the occasional problems when two Macs try printing to the same printer at the same time!
It is a Time Machine server, providing backup resources for our two MacBooks and the iMac. This is particularly important for MacBooks, as it avoids the need ever to physically connect an external backup drive.
I am considering also making it a music server, but I am still only at the “thinking about it” stage!
Sure, it is not being used at anywhere near its capacity, but it performs its task as faultlessly as the software permits!
My Mac mini is from 2012… I’m willing to accept donations!! 
I have it connected to a bunch of external drives, which you might not need since you have a NAS.
If you use Dropbox/iCloud/iCloud Photo Library, it would be a good place to have a local copy of all of your files.
I have my Mac mini running a bunch of Hazel automations. So, on my laptop I just move files to the root of my Dropbox folder, and the Mac mini will sort them where it needs to do.
The Mac mini is also backed up to Backblaze, including all of the attached drives.
Mine also serves as a Plex Server, which will probably work better than a NAS because it has a better processor.
I’ll second a lot of these. My 2011(?) Mac mini acts as a backup for dropbox & iCloud (since it was easy to put in old drives from other places, it has the most internal storage of anything I have - 1 TB ssd and 1 TB hdd). My 2014 Mac mini runs plex, hazel, security camera software, my unifi network controller, scan snap software, printing (incl. a bridge for AirPrint) → basically everything I want running even when my laptop isn’t home. If you’ve got a nice out of the way place you can even buy mounts to hang it on a wall, under a desk, or similar to just leave it there headless.