Failing hard drive

We would need to see the complete EtreCheck report to determine if there are any other issues. Also, you should use DriveDX to test the HD. If the DriveDX report shows ANY errors then that simply confirms your hunch that the drive is failing. Before you do ANYTHING THOUGH please backup the computer to safeguard your data. Use Time Machine to do that, if you already use Time Machine please test it by restore any file some old file on your Desktop would be fine. This just verifies that your Time Machine External Hard Disk is working okay!!!!!!!!! NEVER EVER assume that your backup is okay, that is a good way to lose your data. How do I know this, well I learned the hard way once upon a time.

However if you want to keep the 2017, I would simply buy an external SSD and use that as your startup drive. This will improve performance dramatically and give your old iMac some more years. The external SSD I'd recommend is the OWC Envoy Pro SX and setting it up is pretty easy if you follow the steps in How to setup an external SSD as your startup disk.

This will be FAR easier and less expensive than trying to change the HD in your iMac. These are very difficult to work on and it is simple to brick them if you make one goof!

Your welcome .

I wholeheartedly with rkaufmann87's recommendation of going to and booting from an external SSD and his recommendation of an OWC (MacSales.com) external SSD.

Cloning your internal boot drive to the external SSD with Carbon Copy Cloner is described here: Using Carbon Copy Cloner to Make a Bootab… - Apple Community

I have a 2017 iMac and boot from an external SSD frequently, primarily for beta testing.