Disk repair seemed to function properly, but imac 27” 2015 still gets stuck half way through progress bar when trying to open computer

Try to boot in the safe mode to see if the mac Will boot completely that way

Start up your Intel-based Mac in safe mode

1. Turn on or restart your Mac, then immediately press and hold the Shift key until you see the login window.
2. Log in to your Mac.
3. You might be asked to log in again.
4. On either the first or second login window, you should see “Safe Boot” in the menu bar.

My first guess since it eventually booted is a failing hard drive.

You can try booting the Apple Diagnostics. If the online diagnostics take a long time to boot, then it is unlikely using an external USB3 SSD will be an option. Unfortunately the diagnostics don't detect most drive failures.

You can try running DriveDx to check the health of the hard drive assuming the system can complete the download. If there is any "Warning" or "Failing" notices for the hard drive, then the drive is worn out or failing respectively. This does not necessarily apply to the SSD. If there are no warnings or errors, then post the complete DriveDx text report here for both the hard drive & SSD so we can review them.

If the hard drive is not worn out or failing, then try booting into Safe Mode in case some third party software is really having problems.