Mac Mini (late 2014) running extremely slowly- Etrecheck interpretation & feedback needed please

I don't see any software issues in the report except that you have Adobe Flash installed. Adobe is discontinuing support for Flash in about a week so you should just uninstall Flash by following Adobe's instructions to uninstall it. There is no need to have Flash at all (I've been Flash free for over five years now).

Run Disk Utility First Aid on the hard drive. In fact I recommend running First Aid on the "Container" instead of just the "Macintosh HD" volume. To be able to see the "Container" you must click on "View" within Disk Utility and select "Show All Devices" so that the physical drive and the "Container" appear on the left pane of Disk Utility. Even if First Aid shows everything is "Ok" click on "Show Details" and look to see if there are any unfixed errors. If there are unfixed errors listed, then these errors may be affecting system performance. If there are unfixed errors you will need to erase the whole physical drive and restore from a backup or clone.

If the file system has no unfixed errors, then check the health of the hard drive by running DriveDx and posting the report here using the "Additional Text" icon which looks like a piece of paper like you did with the EtreCheck report.

I don't see anything in the report except that that the Load Cycle Count on the drive is getting high as is the Power Cycle Count. I rarely encounter drives with a Load Cycle Count around 50% so I'm not sure if the drive begins showing performance issues at that point or not. I do know when the Load Cycle Count expires that a drive can become really slow and act strangely. The high Power Cycle Count is a bit concerning though, but may be Ok since the drive does have almost 7k hours on it. There is one Spin Up Time alert so it is possible that the combination of these items are indicating the drive is starting to have problems.

This is a really slow drive anyway. Plus I've heard the APFS file system used on a hard drive makes the system even slower than normal. Also you may be encountering some unknown issue with Big Sur.

You may want to consider upgrading the system to an SSD. You can either directly replace the current hard drive with a 2.5" SATA SSD which will give a nice performance boost or you can upgrade to an OWC Aura PCIe NVME SSD which is 3x+ times faster than a standard SATA SSD. OWC has instructions and videos showing the upgrade process.

https://eshop.macsales.com/shop/ssd/owc/mac-mini/2014

A Crucial MX500 SSD is a good choice for a SATA SSD if you want a decent SSD for a lower price (stay away from the BX500 SSD though).