Probably the best thing you can do for your Mac Mini is to upgrade it to a linear power supply. This does require some modification to the unit. Here's a very good option:
https://uptoneaudio.com/products/mac-mini-dc-conversion-linear-fan-controller-kit-mmk
Then get a linear power supply of your choice, but make sure that it's a 12V linear power supply that provides at least 4.5 amps of current. Many of the popular linear power supplies are limited to 2 or 3 amps. The Keces P8 is an excellent choice. Or you can go for a Teradak from ebay/china.