IMAC hard drive performing as if full after freeing up space

Your Disk and its Volumes are all mounted so no need to do anything.

Have you restarted your mac to see if the problem clears.

You could also try starting the mac up in Safe Mode.

How to use safe mode on your Mac – Apple Support (UK)

Safe mode can take a long time to start up about 10 minutes

be patient. The mac is repairing any errors and clearing out caches.

Run a few tasks in Safe Mode and restart normally, see if there is any

improvement.

Welcome to the Apple Support Communities.

I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Jack

Please go to Disk Utility, and select "View" > "Show All Devices". Select each device (or entry shown), and take a screenshot of it and upload them all here. 

Resources:

Take a screenshot on your Mac - Apple Support

Jack

Sounds like your Macintosh HD - Data partition has unmounted itself.

meaning the mac is only running on the Macintosh HD partition, which is

why you cannot download and install items as this partition/ Volume is

read only and owner by the system.

Open Disk Utility from your Applications> Utilities folder.

Click View in the menubar and select Show All Devices.

When you look at the left hand panel does the Macintosh HD - Data

text look lighter/ fainter than the other Volumes and Drive lettering.

If so that is an indicator your Macintosh HD _ Data Volume has

unmounted.

Highlight Macintosh HD - Data now click on the Mount button.

If it mounts it should now get your mac back to working as you expect it to.