iMac slow

Not sure why you need this:

[Running] net.culater.SIMBL.Agent.plist (? 850e6250 - installed 2019-01-25)

According to one article I found, I consider it a potential troublemaker:

SIMBL (SIMple Bundle Loader) - pronounced like "symbol" or "cymbal" - enables hacks and plugins.

bitTorrent is not doing you any favors either

This is likely what is triggering the "failing hard drive" notice, and I would agree IF you were not running Avast:

Performance:

System Load: 2.07 (1 min ago) 1.92 (5 min ago) 1.82 (15 min ago)

Nominal I/O speed: 3.00 MB/s

File system: 72.51 seconds

Write speed: 21 MB/s

Read speed: 28 MB/s

However, I've seen other posts with EtreCheck reports here recently where removing Avast and other useless anti-virus products increased the read/write scores on a drive that was scoring very low like yours is now.

So, yes, you may need a new hard drive, but make that decision after getting rid of the burden of useless software currently on your Mac.

I’d get an immediate and complete backup, before attempting removal, repair, remediation, or whatever. Your hard disk may be or failing, and the more I/O you toss at a failing hard disk, the less the data you might get back.

Use what’s left of this hard disk wisely, if it’s failing.

Get a backup.

The app crashes imply a low-level file or volume corruption, fallout from having anti-malware installed, or a failing hard disk drive.

The I/O activity could be the anti-malware or other bits—apps such as Oracle Java and Adobe Flash can both generate lots of I/O—or it can be attempts to read an increasingly-failing storage device.

I’d then preemptively replace this hard disk.

If you want to try to keep this Mac for a few more years, install an SSD, and maybe more memory. An SSD will be a big performance boost. More memory will help, too.

Then either restore your most recent (complete, non-corrupt) backup, or install macOS and migrate in your apps and data.

I’d also consider removing Oracle Java, and Adobe Flash, unless you specifically need those. Check the commercial-use licensing on current versions of Java with the Oracle JVM, too Adobe Flash has a long history of security problems, and the whole package is being deprecated.

I agree with removing the programs suggested above. Use the Developer's uninstall instructions.

The 2 places I’ve seen recommended most to buy reliable RAM are below. I have purchased RAM several times, a MacBook Pro battery, and 2 hard drives from Other World Computing and have always been very satisfied with the product and service. They have on-line instructions on how to replace the RAM and internal hard drives.

Crucial

Other World Computing  

This could be the main reason…

From your report: " Failing hard drive - This machine has a hard drive that appears to be failing."

upgrade the memory and remove avast realtime virus scanner stuff

those are the 2 things which result in your poor experience