Everything I'm about to say, please be aware I've haven't paid any bills by doing IT maintenance in about fifteen years, but I have done some of what's described below, on a non-professional basis, in sort-of similar scenarios.
If you are keen to avoid buying a new computer, and want to get your 2014 Mac Mini back to active service (where that means "capable of Logic project work, if increasingly unable to install or run new releases of applications or plugins"), then the key challenges are:
- how to find and install an older release of macOS ? while obscure, this is possible - I've done it within the last two years on my mid-2012 MBP
- how to find and install an older release of Logic ? also obscure, this is (probably) possible
On the assumption that Disk A, the SSD currently installed in your Mac,
is the only copy of your original system and your Logic project data, you should back up Disk A in its entirety, and then have your guy remove Disk A from the Mini to a safe place,
before you do anything else.
The free trial of SuperDuper will let you clone the entire contents of Disk A to a USB disk.
If you already have a USB backup of your Logic project data that you're
absolutely certain has all your stuff in it, you can skip cloning Disk A. But I would do it anyway.
The way I would then attempt to get your Mini back to active service involves installing Disk B, an empty SSD, into the Mac mini, and then installing an older release of macOS as if I were setting up a fresh system from scratch. I am reasonably confident that if you are able to
re-install the macOS release you were originally using on the Mini, and then sign in to your Apple ID, the App Store will then allow you to download the most recent release of Logic that is compatible with that macOS release. At
that point, you should be able to import the Logic project data from your USB backup.
Because project work is precious and storage is cheap, I recommend a short-term shopping list of
- USB Disk 1, an external USB disk that's big enough to copy the entirety of Disk A (e.g. the one with the attempted upgrade, that's in the Mini right now)
- Disk B, a new internal SSD, which will let you try the routine below on a disk without anything precious on it; if the routine succeeds, you can then import the precious stuff from USB Disk 1
- USB Pendrive 1, a decent, new, 16GB or larger USB Flash pendrive -- this is to create a bootable USB installer disk for the old macOS release
The rest of this is about attempting to get your Mini into active service as a Logic system again.
- preserve the original Logic project data - create a clone of Disk A's data on USB Disk 1
- have your guy remove Disk A to a safe place, and install Disk B into the Mini
- you can download installation images of older macOS releases from this support page (although you may need to use Safari to get the downloads). That linked page also has instructions (at the bottom) for creating a bootable installation USB disk, which is what USB Pendrive 1 is for. Follow those instructions carefully. Plug USB Pendrive 1, which now holds a bootable installer for macOS, into the Mini once Disk B has been installed in it. The Mini should then be able to boot off USB Pendrive 1, and this should enable you to install macOS onto Disk B.
- Once that's done, remove USB Pendrive 1, boot the Mini off Disk B, do all the "Hello!" shit and be sure to sign in with your Apple ID.
- Once you're signed in, the App Store should allow you to download an older (i.e., compatible) release of Logic (open the App Store, go to Account and then Purchased)
- At that point, you should be in with a pretty good chance of being able to import your project data from USB Disk 1. But I haven't used Logic since it was available for Windows, so I can't offer any advice here, I'm afraid.