Add me to the list - 2019 iMac running slow, etrecheck attached

As you guessed the HD is failing. Before you to ANYTHING please back it up using Time Machine, please read Back up your Mac with Time Machine. Operating ANY computer without a backup plan is a 100% guarantee of losing data. Backing up will require an external HD, many experienced users on these forums recommend the OWC Mercury Elite Pro due to it's reasonable cost, high quality, great support and ease of replacing the HD when it dies.

+1 on making a backup. To start, you need an external hard drive. Here’s a few excellent ones sold by Apple: 

• LaCie Mobile Drive 4TB External Hard Drive USB-C - Apple
• LaCie 4TB Rugged USB-C Portable Hard Drive - Apple

You can then follow the steps in this Apple Support article to setup your Time Machine backup: Back up your Mac with Time Machine - Apple Support. 

Also, moving forward, I recommend that you use iCloud in addition to Time Machine to sync all your files and photos to all your Apple devices. Although iCloud is not a "true" backup service, iCloud can also act as an emergency off-site backup, but this is why iCloud should be used in addition to Time Machine. Here are some resources on setting up iCloud for Mac:

• Upgrade to iCloud+ - Apple Support
• Set up iCloud Drive - Apple Support
• Add your Desktop and Documents files to iCloud Drive - Apple Support
• Set up and use iCloud Photos - Apple Support

After you setup iCloud, turn off "Optimize Mac Storage" in System Preferences > Apple ID/iCloud. That way, all changes made to files in iCloud are also backed up to Time Machine. If you have this setting on due to a limited amount of storage on your Mac, that is fine. Just know changes won't be backed up to Time Machine, only iCloud.

Also, what external SSD did you buy?

Jack

You can get DriveDx to more thoroughly test the HD. If it detects ANY errors, then the drive should not be trusted.