Mac won't boot, need additional tips

iMac (Retina 5K, 27-inch, Late 2014) with 3TB Fusion drive.

My wife's late 2014 27" 5K iMac crashed/froze last night. She rebooted and went to bed as it was past midnight. She came down this morning and it was on the Apple logo with the progress bar about 50%. She tried rebooting and same thing.

I have tried the following:

• Booting into safe mode - Progress bar until about 50%, then no progress
• Booting into recovery - Same deal, progress bar until about 50% and then no progress
• Reset SMC - Pulling the power cord and all peripherals, holding the power button for 10 seconds. Same result
• Reset NVRAM/PRAM - Same result
• Booting to Internet Recovery Mode - Globe progress completes, moves on to apple logo with progress bar and actually gets through it! But then it freezes on the language selection screen (spinning beach ball until even that freezes) - Did this 3 times and the same result every time.
• Created bootable media - Downloaded Big Sur to a USB drive and created a boot drive from it, I can get to the boot loader screen and select the USB but then apple logo and progress bar, only progresses to about 25%
• Ran diagnostics as per u/CraigJDuffy suggestion. No issues found, Ref ADP000
• Tried to boot in target disk mode, doesn't show on connected computer and, in fact, crashes connected computer

I'm at my wits end, I don't know what else to try. It doesn't even appear that I can take the nuclear route and reformat it. I'm not even sure what the issue could be. You'd think that if it was a logic board issue that it wouldn't even get as far as it does.

But if it's a hard drive issue, why wouldn't it boot from the USB? Even if I went DIY and pulled it apart, I'm no closer to knowing what the issue is.

We don't have an Official Apple Genius Bar/Service Center nearby (Stavanger, Norway) which means we'd have to ship it out. Don't even want to know how much that would cost.

I'm hoping anyone can reach out with any other solutions that I may not have tried. I'm desperate. I'm not opposed to nuking the data, we have everything backed up.