Fusion Drive Conversion

For half of the old system, yes.

Apple's Fusion drive consists of two separate components linked by software:

• a small NVME solid-state drive (SSD) hidden on the logic board
• a large-capacity conventional mechanical hard drive—a 3.5-inch version in 27-inch iMacs— in the drive bay.

Once removed from the computer, the software link that makes Fusion faster would be lost, but you can still put the big mech drive in an external enclosure. It would be no faster than any similar external mech drive , but fast enough.

After struggling with most "name-brand" external enclosures, I now only use these for my 3.5-inch external drives:

https://eshop.macsales.com/item/OWC/ME3NH7T00/

We have three of those enclosures serving two iMacs, and they have been brilliant.

I also strongly recommend the OWC solution, DO NOT remove the HD from your iMac it is ancient and very prone to failure due to its age. Also the OWC solution is inexpensive, simple to use and the support from OWC is great. In short there is no reason to NOT use them.

You could take the hard drive out of the 2014 iMac, and use it for your backups, but I wouldn't. It would be a lot of work to salvage an old drive.

That Mac has a very nice 27" 5K Retina display, USB 3.0 ports, and upgradable RAM. It can run Big Sur – which is outdated now, but enough to run current versions of FireFox and LibreOffice. There might well be some school or charity or senior center in your community who could make use of the machine for running Firefox and LibreOffice if you don't have any use for it any more.

Whatever you do with it, you might want to read:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

P.S. - I would suggest backing up your photos to at least two places. Backup drives – even new ones – can fail or be burned up in fires or be stolen. When you've lost access to your original data is NOT the time you want to find out that you cannot count on your one and only backup.