Desperately need help restoring fusion drive on a 2019 iMac.

To Fuse the drive please click How to fix a split Fusion Drive - Apple Support

If that does not work, then take the computer to a third party Apple Authorized Service Provider and ask if they can assist. To begin finding one in your general area click the Support link on WWW.Apple.com (it's on the far right).

If the erase fails, then try erasing it again. I have seen Disk Utility fail to erase a drive on the first attempt, but will be able to erase the drive on the second attempt made immediately after the first failure. If a drive won't successfully be erased after several attempts, then it usually indicates a hardware issue. Also, try erasing the SSD while booted to the macOS installer.

Try running DriveDx (free trial period) to check the health of the SSD and posting the complete text report here using the "Additional Text" icon which looks like a piece of paper on the forum editing toolbar. You may as well post the health report for the Hard Drive as well.

Also, when posting pictures please make sure they are oriented correctly. You can easily rotate pictures on an iPhone/iPad by using the instructions in the following Apple article (I'm sure Android devices have a similar option):

Crop, rotate, flip, or straighten photos and videos on iPhone - Apple Support

For a M4 MM, I'd recommend budgeting about $1350. I'll break it down....

• M4 MM with 16GB RAM and 1TB SSD (NEVER GET LESS STORAGE THAN YOU NEED!!!!!) $999. A $700 M4 MM is kind of short sighted.
• 27" 4K display about $350. (Check on Amazon, they're easy to find.)

The good news is you can finance the MM for no interest for 12 months if you buy directly from Apple. 12 payments of approximately $85 per month. If you go that route it makes the purchase much more palatable! If you don't know how to do that you can call Apple's on-line store or if you have an Apple Store in your general area they can assist.

Make sure that both the internal HD and SSD are not mounted. You can do this in the Disk Utility GUI app.

Theoretically you should then be able to launch the Terminal app and issue the command to reset the Fusion Drive. If you still have problems, then I would go back to Disk Utility and erase both of the internal drives, before trying the Terminal command again.

My question is:

If you really want to save your computer (meaning not wanting to replace it) then why don't you simply take it to a qualified professional that knows what they are doing? In other words please take it to an Apple Authorized Service Provider rather than attempting to do-it-yourself! Don't tell us you are trying to save money, we read all the time of incompetent do-it-yourselfers that totally mess up their system rather attempting to save money, when they end up spending far more by trying to do-it-themselves and then making a total mess.

At least take it in, get an estimate and then you can make a more educated decision!!!!!!

The DriveDx reports were not properly attached. You need to copy the text from the DriveDx report and paste it into the "Additional Text" box. You cannot copy & paste the file or drag & drop the file here.

Definitely curious about the SSD since it seems there may be some issues with it.

Very interesting health report for that SSD. I've never seen one quite like it before. Parts of the health information appear to be corrupted and/or out of bounds. No wonder you are having issues with that SSD.

If that SSD was installed in any older Mac, then I would suggest trying to perform a reset of the SSD by using the SSD's built in hardware Secure Erase feature which besides wiping the SSD completely, it also will reset the SSD to factory defaults. I have "fixed" many SSDs this way which were encountering odd glitches/issues. However, the only way to do this by booting the computer with Linux & using a special Linux command line utility to activate the SSD's built-in hardware Secure Erase feature. Unfortunately Linux support of the USB-C Macs is very poor....I don't know if or how well Linux may function on this 2019 iMac. Plus there is a chance of completely bricking the SSD although this SSD is certainly not able to be used in its current state.

Of course your new internal Hard Drive is completely healthy with just a bit over 100 hours of use.

You will either need to run macOS from the slow internal Hard Drive, or you could install macOS to an external USB3 SSD so you run the iMac from the external SSD instead. The external USB3 SSD will be much faster than booting from the internal Hard Drive (assuming you have a good quality external SSD since many SSDs these days can be quite slow when writing....sometimes making a Hard Drive look fast).

Sorry for the bad news. It is also one of the few times DriveDx has actually revealed a bad SSD since most SSD failures are due to the SSD's controller which has no health monitoring.

I noticed your trying to format an SSD. They have limited life and you don't manage it the same way that a HD would be used. Using a new disk partition may make the drive unusable. You can't defrag an digital SSD drive. If this drive is removable then try and use a linux disk recovery app on it like kde-partition manager. If it's formatted as APFS the only macOS disk-utility can fix it. From your screenshots. I'd say you wiped your drives' containers so try to create a new one with disk-utility first. Avoid format as SSD don't do well with these old tools. I have bricked a few trying to recover the partition with format. good luck.