I see several things, but the area solutiosnteh let you keep this computer a little longer. AS for probelms, the foremost is the sklow factory mechanical hard drive:
Drives:
disk0 - APPLE HDD HTS541010A9E662 1.00 TB (Mechanical - 5400 RPM)
Internal SATA 3 Gigabit Serial ATA
It is a low-performace laptop-class drive. That said, if healthy and without software interferences, it should post write/read scores of 70-80MB /sec.
Your scores:
Performance:
System Load: 4.47 (1 min ago) 2.96 (5 min ago) 2.67 (15 min ago)
Nominal I/O usage: 0.50 MB/s
File system: 51.72 seconds
Write speed: 56 MB/s ⚠️
Read speed: 49 MB/s ⚠️
That could be a failing drive OR it could be some useless software you've installed. So:
1) completely remove all Intego products
2) completely remove CCleaner
Catlike, Macs clean themselves and I've seen plenty of EtreCheck reports were removing a useless cleaining app improved drive scores.
You have third-party drive management software from both Western Digital and LaCie. You do not need those and they will extract a performance penalty if left.
Also test with your external drives disconnected. I see a lot of issues with WD Passport drives. And your Passport drive is so full that it could slow tasks.
Those steps will help but the not fix the fact that the hard drvie is simply too slow for newer macOS versions. The cost-effective solution allowing you to keep the current computer is to use an external USB3 drive with an SATA 6G SSD inside set as your boot volume. This can be done at home by the average user who has read this excellent user tip:
Use an external SSD as your startup disk … - Apple Community
You do not have the remove the internal drive and can still use it for basic storage.
That solution can be well under US$100 and your speeds will go from a best-possible today of 70-80MB/sec to 400 MB/sec. That will make your Mini feel like a new computer.
The other plus to this solution is that, when you do replace this Mini, the drive can still be used for additional stoarge on the next computer. Retained value is agood.