Sluggish iMac

Your mac only has a 5400rpm hard drive, and that is the weakest point.

All recent versions of macOS perform a lot better if your system drive is an SSD.

8 GB is reasonable, but Chrome is a huge resource hog, and if you happen to use a lot of tabs it will suck up a lot of memory.

If your system then needs more, it will use swap space on the drive - and there again the slowness of the drive shows, painfully.

There are two things I can suggest:

1) to mitigate this, now, try using Safari or Firefox browsers. They are usually nimbler and use less memory.

Chrome, although seen by many as a sort of "standard", is a very poorly programmed browser, especially in macOS.

You can see more about the damage it can do here: https://chromeisbad.com

2) to really get yourself a far more responsive, almost "new" mac: get an external SSD, and install the OS on it. Use the internal HD as additional storage.

You will see your boot times greatly reduced, applications launch almost instantly, and when the need for swapping arises, it will be much less noticeable.

Oh, and SSD are now quite more affordable than they used to be.

You can get a 250GB with an external case for less than $60, or a 500GB for under $90.

The biggest issue, as per the Limited Disc Access Report, is the slowest Rotational / Mechanical 5400 RPM Drive. No matter how clean of an OS and Third Party software used on the computer - that is going to hold back the unit from a meaningful performance. Alternative is to purchase a Reliable External SSD Drive and install Big Sur to that drive. Make the External Drive the Start Drive. Just be certain the external Drive is larger enough to you storage needs.

Right now software wise - TunnelBear as a VPN. will slow the Internet down by 20 - 30 % of the best possible speed if it is not used at all. Factor in  Commercial VPNs should only be used if Mandated by Corporate Head Office for working remotely. Otherwise - ones Privacy and Security touted by the VPN is suspect.

Lastly - the computer is running, Big Sur 11.4 which is out dated. The current version is 11.6. Caution on this 11.6. From what am seeing often times, is the 11.5.2 is Required Before the 11.6 is permitted to download and install.

In principle one can use SuperDuper! (or CCC... maybe) to clone the internal to the external.

As P. Phillips said, though, this is harder in Big Sur (only recently did SuperDuper! offer a Big Sur version).

I strongly suggest:

1) Get a USB thumb drive and create an installer disk (you could do it from Recovery, but this method is probably less error prone):

1a) See https://support.apple.com/en-us/HT211683

1b) See hhttps://support.apple.com/en-us/HT201372

2) Start from this installer drive. Choose Disk Utility.

Completely erase the external drive, choosing APFS with a GUID Partition Map.

3) Quit Disk Utility and start the installer

4) When asked if you want to migrate, select "From another Mac, disk or Time Machine backup".

THEN: you could check all the boxes and make your external a perfect replica of your internal,

BUT: I recommend you check only the user accounts (check everything pertaining to your account and possibly other users, if any); do NOT check Applications, Settings or Other Files.

While this may need you to then install some applications, it prevents you from bringing old software to the new drive, and let you start fresh and healthy.

Well, Big Sur is more Locked Down from Previous versions of macOS. So using a Cloning Software may or may not achieve the desired results.

It may work better to installed Big Sur from the Full Installer and point it to the External Drive as Destination. Then is System Preferences >> Startup Disk >> Make the External Drive the Startup Disk.

Once that is done and works - one can use Move your content to a new Mac which in this case would the the External Drive.

Am open to additional Advise from @Luis S on this thought.

@ Luis S. idea of a Bootable Installer on Thumb Drive seems better and much more Direct to the end result desired..