Possible suspects:
AppCleaner -- many Mac "cleaner" apps conflict with the MacOS or have been traced to slow performance
NordVPN -- this could also interfere, and in fact it shows up as a significant resource user, which is odd for a VPN.
I suggest completely uninstalling both of these and retesting. An uninstaller from the vendor is best, usually.
You have a fusion drive, which is 99%+ a mechanical hard drive, with a tiny SSD. Mechanical drives are VERY slow under APFS. (Your read/write speeds shown in Etrecheck are ok, but those are with the tiny SSD, not the much larger HDD.)
https://bombich.com/blog/2019/09/12/analysis-apfs-enumeration-performance-on-rotational-hard-drives
https://eshop.macsales.com/blog/43043-using-apfs-on-hdds-and-why-you-might-not-want-to/
Your fusion drive might be failing. You can check it with DriveDX (hardware health).
You can go to an Apple Authorized Service Provider and have them replace the fusion drive with a complete SSD. I did this with our late 2015 iMac and it is extremely fast and responsive now.