My iMac is going slow 'beachballing' and crashing

The only huge red flag I see is that the installer log shows MacKeeper, one of the biggest scams ever foist upon the World of Mac. I don't see it in the other areas of the Etrecheck report; however , if it is installed you must get rid of it.

Something else to try: Disconnect all USB devices (except for mouse and keyboard) plus the WD FireWire drive, restart, and see if the computer runs better. I am suspicious that the WD MyBooks (you have several?) are causing issues. I had trouble with them on both the pre-Intel Macs and current Macs. The drives inside the enclosures are good but WD applies an odd formatting scheme that I believe is part of the problem. Also their enclosure electronics are OK but not as good as some. The other part of the problem is that their "SmartWare" is badly misnamed. It is a very poor product. So...

If the computer runs better without the extra USB stuff, reintroduce them on at a time (you have a lot of devices) and test, looking for any degradation in performance. Connect the USB MyBooks next to last and the FireWire MyBook last.

If the drives seem to trigger the problems and you have a way to offload their contents for maintenance, do so and then use Disk Utility to erase and reformat the drives as Macintosh Extended (Journaled), then reload the files from where they was moved during maintenance.

If reformatting does not completely fix the issue then you should consider a better drive enclosure. I've personally had serious issues or have helped others with issues with EVERY "name-brand" external drive that go on sale every weekend. They are low-cost devices to get people in the store and some are also low-quality. After the all my external drive travails I now buy only empty enclosures from Other World Computing into which I install "bare" WD Black premium drives. I no longer have issues with external drives slowing my workflow.

For our desktop Macs I use this model:

OWC Mercury Elite Pro USB 3 With USB+1 HDD... at MacSales.com

and this model for our notebook Macs:

OWC Mercury Elite Pro mini 2.5" Bus-Powered... at MacSales.com

You may find some benefit from not leaving USB flash drives ("thumb" drives) connected all the time. There is some evidence that older flash drives of questionable quality can cause a small reduction in performance.

Disclosure: I have NO relation with OWC other than that of a satisfied customer.