My 2017 iMac has become extremely slow, and is barely usable. It takes forever to open anything etc. I only need it mainly to prerecord a radio show weekly via Zencaster on line. Otherwise, I use my iPad /iPhone.

Your drive's read and write speeds are about what you'd expect for a 5400 RPM HDD ... quite slow. And that will affect system response times.

You need to uninstall CleanMyMac and all similar "helper/fixer" apps as they often disrupt system operations.

You can't improve on the HDD, however, you can boot from and external SSD to have better response times. My 2015 with external boot drive read at about 480 MBps. That's almost 6x your drives read speed.

It gets worse. The other issue is right here:

iMac (21.5-inch, 2017)

Status: Supported

iMac Model: iMac18,1

2.3 GHz Dual-Core Intel Core i5 (i5-7360U) CPU: 2-core

You have the crippled educational/institutional iMac made for bulk buyers looking for an attractive price. It has a low-performance. 2-core laptop-class processor.

The standard consumer 21.5-inch iMac 4K for 2017 that cost only US$200 more had a proper 4-core desktop-class processor. Even the slowest of the 4-core models ran 50 percent faster than yours for roughly an 18 percent increase in purchase cost.

These are benchmark scores from the MacTracker database. Yours is shown above the regular consumer version:

Still, setting up an external USB3 SSD drive as the new boot volume will drastically improve the feel and perceived speed of what you have now for, in most cases, around US$100.

It does not require professional installation or opening of a computer case that Apple did not design for user access to the innards.

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

If "splash out" mean "purchase", that's your call. I have no dog in this fight.

My comments of helper apps stands.