iMac accepts login and gets halfway through loading

This article is meant to cover this situation:

If your Mac starts up to an Apple logo or progress bar - Apple Support

When the Apple logo appears, that signifies that a disk that appears to be a boot disk has been found. But if the boot up stalls, then something is wrong with the MacOS installation, or there is a hardware fault.

You could try booting into safe mode.

In the above article, it is suggested also to reinstall the MacOS. Have you tried that?

A hardware fault would need to be evaluated by an Apple Authorized Service Provider. What model and year is your iMac?

If all previous suggestions fail Boot into the Recovery volume (boot with the Command + R keys held down  - Intel Macs) or  (How to Boot an M1 Mac into Recovery Mode), select Disk Utility and run First Aid on all available volumes. Then try to reboot normally.

Booting into Safe Mode is the first thing I'd try.

Have you tried starting in Safe Mode followed by a normal restart. If you have any anti-virus or cleanup apps install these could be messing with your system and should be uninstalled.