iMac running slow after a few years of no use

RAM is not your issue. Adding RAM will have zero effect on your real issue, which is right here:

Drives:

disk0 - APPLE HDD HTS541010A9E632 1.00 TB (Mechanical - 5400 RPM)

Internal SATA 3 Gigabit Serial ATA

These are slow laptop-class drives, and even though yours is actually running faster than most, it is still at 1/5 to 1/4 the speed of the cheapest solid state drive.

I agree 100% with the recommendations for evicting your CCleaner. Early versions had the nasty habit of deleting important Mac system files. Cat-like, Macs have cleaned themselves automatically for nearly a quarter-century. THEY NEED NO OUTSIDE HELP! Adding ridiculous fear-ware only creates conflicts with the elegance for which you paid Apple good money to build into macOS.

Unfortunately, removing CCleaner alone will not make your under-spec entry-level hard drive faster.

The most cost-effective way to have this computer feel frisky is to use an external SSD boot drive.

Use an external SSD as your startup disk … - Apple Community

This is an at-home, no pros needed, upgrade. The least expensive USB3 versions are under US$100 and will do 400MB/sec—4x faster than your current drive. Your 2019 iMac has the more expensive—but far faster—option of a Thunderbolt SSD. Those screamers can do over 2000MB/sec.

The advantage to the solid-state external is that, after your current computer has run its course, you still have a drive that will work on a new model for era storage. Retained value.

I concur, CCleaner needs to go. Otherwise, the amount of RAM is a bit low (not a huge issue) and it looks like this computer only has a mechanical HDD, not even a Fusion drive (I can't see any SSD in the etrecheck report). That's going to under-perform with APFS as the file system.

I agree wholeheartedly with varijak paw regarding CCleaner. It's totally unnecessary and can interfere with system and application performance. Uninstall according to the developer's instructions.

You can check to see if you've removed all of the supporting files by downloading and running the shareware app Find Any File to search for any files with the application's or the developer's name in the file name.  For the Cleaner software you'd do the following search(es): 

1 - Name contains ccleaner

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

If you get warnings that the file can't be deleted because it is in use or used by another app boot into Safe Mode according to How to use safe mode on your Mac and delete from there.

Note:  if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode.  This makes it act as a wired keyboard as will assure a successful boot into Safe Mode.

Be aware that the iMac model you purchased was the slowest iMac Apple offered at that time: i3 CPU, 8 GB RAM and a 5400 rem HDD.

Others may spot other issues, but first, I'd suggest that you get rid of CCleaner. That's not going to be helpful and can cause damage unless you're extremely careful, though from the EtreCheck report that doesn't appear to be an issue right now.

Looking at the processor usage, mds_stores is running hard. That's the indexing process used to generate Spotlight results. It may be just that since the Mac has recently been significantly updated and new software installed that it's just taking a while to get everything indexed, and it'll be slower while that's going on. You may just want to wait a day or so to let that indexing complete and see if performance improves.

Again, others may see other issues and have additional suggestions.

Regards.