I need information for recovering data from a dead iMac fusion drive

Hi, I had a late-2014 iMac 5k with a 1gb Fusion Drive. The power supply died and I was told by multiple repair persons (including Apple Genius Bar) that this could no longer be repaired as the parts are no longer made. I am trying to salvage the hard drive, but I have questions about how to do this effectively. (While I did make regular backups to an external hard drive while the computer was functioning, I’ve found that some of the most recent backup is missing some very valuable files and I’d like to try to retrieve those.)

First, I don’t have hardware specifications about the hard drive size to find a compatible enclosure (which I’d like to do before I crack open the dead iMac).

Second, as this is a Fusion Drive, my (admittedly fuzzy) understanding is that only the HDD portion of the hard drive is removable? How would one recover any data that was written to the SSD portion of the hard drive?

Alternatively, am I just doing this the wrong way? Is there an easier way to acquire the data from this computer?