I am running High Sierra from a late 2009 MacMini (3,1) and I am having no issues at all.
I actually stopped using this little Mac Mini when Plex stopped support of the El Capitain (I think?) and I put it aside.
When I discovered dosdude1 little trick I wasn't very hopeful it would work, but the first thing I did was to transfer the disk image to a 250GB ssd I had lying around. That alone made the little mac zip like a little trooper. Big difference, but still the OS could hardly load web pages, so I went ahead and installed HighSierra.
I am really happy with it. Some things, like "pages" and "numbers" I can;t use, but I downloaded Libre Office and that works great.
Garageband seems to be working fantastic (I am having issues with my sound cards, but I am far from having tried all of them) and even after installing a bunch of browsers (Vivaldi) and some other programs I run also on Windows (like Roboform for my passwords, Google Dive, etc.) it's still surprisingly quick. A lot quicker than my wife's far newer windows laptop that shipped with Win 8 but in Win 10 feels like it's in a vat of molasses even with an SSD and 8GB of memory.
My Mac Mini has 3GB of memory and it's just a bit slower than my Win 10 laptop with 16GB of memory and a much faster Intel chip. I am actually incredibly surprised at how well it works. It surpassed even my most optimistic expectations. My plan was to turn it into a Plex server for watching movies and music tutorials, instead I am running it like a fully fledged Mac.
I'd suggest you try to install High Sierra and max both your hard drive (buy a good SSD) and RAM. I frankly cannot wait until I buy new memory and see how it performs.
The one thing that makes me sad is that eventually this too will die because Apple is evil that way. Frankly, the best thing I did professionally was to move from a Mac Pro based workstation to a Windows one. My last machine lasted me 10 years and cost me around $3k. Two Months ago I rebuilt a new workstation recycling as many components as I could (like the case, fans, power supply, etc.) from my old one and now I have a absurdly fast and reliable beast with 128GB of RAM, an older GPU that does the job for now and acres of disk space. I can keep opening Adobe apps and browser windows all day and barely notice the CPU (AMD Rayzen) break a sweat. All for around $1200. I figured an equivalent Mac pro would have set me back thousands and one day would die not because the hardware was old in itself, but because Apple decided my configuration had grown too old. Bollocks!
As happy as I am to have revived my little mac, and as familiar the MacOS is to me, I simply could not afford to run my work on MacOS the way Apple is managing it these days. And to think that 20 years ago even the crackers stopped seeding mac torrents in order to help Apple get back in the game. Remember those days?
Anyway, good luck with your Mini. Let us know if High Sierra is good enough for you and if not why. I need to know for myself as well.
