I lost my data from a Fusion Drive

Probably not. A Fusion Drive operates as a single virtual volume spanning both the SSD and the "spinning" hard drive, and if you just replace one component, that breaks the volume and generally renders the data inaccessible. And now, of course, since you erased the original SSD, you can't just put the hard drive back and rebuild the Fusion Drive.

It's possible that a file recovery utility such as DriveDX might be able to get at least some of the files back from that hard drive, but unless someone else here knows of an alternative method (which is possible; I'm far from being completely knowledgeable about Fusion Drives), then absent a backup that you made prior to replacing the SSD, that data is quite possibly lost.

Regards.

DriveDx is not a data recovery app, but an app to check the health of drives.

You can try using a data recovery app such as Disk Drill or Stellar Data Recovery to see if anything can be retrieved.

Edit: More than likely the hard drive is failing. If so, then you need to be very careful trying to recover the data since any attempt to retrieve data from a failing drive will make the drive failure even worse...to the point where even a professional data recovery service will be unable to recover any data. If the data is critical, then I highly recommend checking the drive's health and contacting a professional data recovery service. macOS & most data recovery software is not able to handle the errors produced by a failing drive.

In the future make sure to always have frequent & regular backups of your computer and all external media (including the cloud) which contains important & unique data. There are a lot more new ways to permanently lose access to the data on the internal SSD of recent Macs due to all the hardware, software, and security changes. Even traditional hard drives are getting harder to recover data when they fail & almost always requires a professional & their expensive tools & software.

Thanks, HW. I had a brain freeze and was thinking of Disk Drill and for some reason posted DriveDX. Clearly a "senior moment".

Cheers.