Every upgrade puts a device further away from the original OS, iPhones, Minis etc. and a device never works as well as it did with the original OS.
That said, if you do not need to use older (often free) 32 bit programs, installing an SSD (i.e. 128 or 256gb) only for the OS and programs would make up for some of the lack of processing power in your 8 year old Mini. You can leave the HD in place and clone the OS and programs to the SSD, then install a newer OS on top of it. Then use the old internal HD only for your personal files/storage.
I bought a 2013 MacBook w/SSD w/ 10.14 - it was fast, but even faster when I downgraded to OS 10.12