Mac Mini (late 2014) "hard drive appears to be failing" -EtreCheck

Your read/write speed indicated the "possible failing" even within 2.5" 5400HDD's realm.

[Write speed: 62 MB/s]

[Read speed: 82 MB/s]

Most of all -- I hope you have a very good backup (you did) or 2 TimeMachine backups, and maybe a clone (Carbon Copy Cloner or SuperDuper) for extra security. One of my backup TimeMachine died during restore process (still had nightmare from that experience) from my distant memory.

There are 2 options (I omit the Thunderbolt 2 external drive route)

1. external USB-3 SSD - you can just plug the USB-3 SSD and use clone (mentioned above) your original HDD. Later, you can go into "System Preferences" --> "Startup disk" to assign the external SSD as the future Boot-Drive.
2. Internal SSD - you can search on the web or OWC for "how to" process. 2014 MacMini requires a special tool (especially T6 Security screwdriver). You can either choose "clone" via USB first/test as external option - then move into internal SATA.
3. You decide to use TimeMachine and re-install the MacOS -- you better to have a USB bootable MacOS if you choose this method. Although this method is my go-to approach, but it did take longer time.

Most user here will recommend OWC for the SSD -- not only the positive feedback, but also the warranty OWC would provide -- again, electronic parts would still fail. I am a Crucial user, mainly for the support of firmware update.