Can't log onto 2017 iMac 27"

If your Mac doesn't start up all the way - Apple Support describes all available options, some of which you already tried.

"Learn what to do if your Mac starts up to an Apple logo or progress bar" is probably the most applicable since you are unable to progress beyond entering your login credentials. Proceed with those instructions, which lead to more, which may in turn lead you into an infinite loop with no apparent resolution.

The penultimate step is to erase the Mac and recover its content from a Time Machine backup, and if that fails contact Apple Support. That's all you can do.

Use Disk Utility to erase an Intel-based Mac - Apple Support

If you encrypted its startup volume with FileVault then there is no need to erase anything. Just drop it off at an Apple Store or AASP. Its data are utterly useless without its encryption key (normally, your login credentials) which are also erased upon erasing the startup volume. It's a brick.

If you are unable to erase it and you did not use FileVault the extreme but technically accurate answer is to physically remove the internal storage device (hard disk drive and / or flash memory) and subject it to irreversible mechanical destruction as described here: Guidelines for Media Sanitization - NIST

Specifically,

• Disintegrate
• Incinerate
• Melt
• Pulverize
• Shred

I would say that if you want to erase it, you should take it out and, depending on its connector, get the connector-to-USB adaptor to work with the drive, and then take WIndows to it and use it in a different computer like your great-great-great-great-great-grandmother's computer (the Harvard Mark I).