iMac running slow EtreCheck says below Average with 128GB Ram

I do not see any of the normal suspects so I would focus on a hardware issue. I would run DriveDX reports on all storage devices. Your computer originally came equipped with a Fusion Drive and you apparently added an OWC external SSD but left the original 1TB HD installed. I suspect that drive may be failing or may have failed already.

Run DriveDX on the individual components, NOT THE FUSION DRIVE. Also, if you have any external HD's attached (like the external SSD) test those as well.

If any errors appear on any of the drives, that indicates a failing or failed drive which must be removed from the equation.

The drive is still inside and rotating. It may or may not bee the issue, perhaps the most logical next step would be to take the computer to your local Apple Authorized Service Provider to have it professionally diagnosed .

These data:

Performance:

System Load: 1.72 (1 min ago) 1.92 (5 min ago) 3.59 (15 min ago)

Nominal I/O usage: 1.02 MB/s

File system: 17.29 seconds

Write speed: 1434 MB/s

Read speed: 2718 MB/s

show your Fusion drive is running well above expected speeds and is quite healthy. Is the OWC tied is as part of the boot routine? If not, try disconnecting it to see if it is somehow contributing to slowness

When you added RAM, from whom did you buy, and want make/model?