To pile it on: your actual drive performance numbers.
Performance:
System Load: 4.86 (1 min ago) 2.68 (5 min ago) 2.16 (15 min ago)
Nominal I/O usage: 0.88 MB/s
File system: 18.90 seconds
Write speed: 169 MB/s ⚠️
Read speed: 697 MB/s ⚠️
Those are horrible. 🤮
A proper Fusion drive in your nicely-appointed iMac should do Writes between 600-900MB sec and Reads up to 2000 MB/sec. Here's the good news:
— An 18-second file system score shows your drive is still healthy.
— Both a too-full drive and CleanMyWallet can account for the slowness.
— Both are correctable.
TIP: My daughter had similar too-full issues with her SSD-only 2019 iMac. I had to boot into safe mode to even start to reduce the clutter.
I see piriform.com references. Although CCleaner (piriform.com) is not loading, it is a bad actor as well. You need NO cleaning apps! All hurt performance., CCleaner was ported from Windows and early verisons had the unpleasant habit of removing critical Mac system files as " suspicious. Sloppy.
I would evict any WD software. Even WD techs have told callers that those are not needed on a Mac.
Also, use the top part of this Apple article to see if the two componets of your Fusion drive have "split," that is, lost track of each other:
How to fix a split Fusion Drive - Apple Support
Etrecheck does not suggest that it is, but I've seen a handful cases here where a split was not reported. If it is split, follow the instrticutosi that article.
👉🏻 I would NOT recommend reinstalling yet. I see enough issues with that being an issue on struggling Fusion drives that I consider it "Last Desperate Act." I would rather see you:
— first confirm your Fusion drive is not split, then
— eject the Useless-ware, and then
— address the overcrowding.
Then we can talk reinstall.
FYI: the last macOS I found needing reinstalling to fix serious issue was 10.2 Jaguar which still felt "beta" to me. 10.3 "Panther" in 2003 fixed the issues and I've never had to reinstall since, and I have maintained a fair number of Macs for family and friends.