Here's another possibility: The internal backup battery may need to be replaced. It isn't user-replaceable in the Mini. I mention this because I had a sleep issue with my Mac Pro wherein it would not wake from sleep. I had to do a forced shutdown and restart. Replacing the battery cured it.

Go here for links.

Jon

macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365