This is just what I am looking to do. Can you explain a little more detail how you went about setting up the Fusion drive and what size PCI-E and make / model did you use?
I just googled: how to create a fusion drive
and i ended up using this site:
http://benscheirman.com/2013/11/creating-a-fusion-drive/
So i first installed the mac mini, so i have a working os, then used carbon copy cloner to copy the working os from the mac mini to a usb stick
I bought my gear from ifixit, data kit (for cable) and a tool kit with the new T6 security driver,...
Use a few youtube video's to assist with the teardown:
https://www.youtube.com/watch?v=4SaVDTq353A
https://www.youtube.com/watch?v=2C-a71PSPgs
(you will be missing a small cap + screw to put over the new cable but the cable seems to snap into place firmly enough)
After that i wanted to see if the drive was working so i booted from the usb and formatted the hdd to see if i got a working (Samsung M9T) 2TB working drive. (worked fine)
Then i followed the command line instructions and all from this site:
http://benscheirman.com/2013/11/creating-a-fusion-drive/
But i got errors that he could not identify disk02 or something, after a while i gave up and formatted my pcie flash drive (what do i have to loose) and when i did that disk utility asked if i wanted to restore the drive, so i clicked yes and disk utility created the fusion set by itself! Victoryyyyy!
Then i tried installing the OS again through internet recovery, that also failed, then i booted from my cloned usb stick and cloned the usb stick to the 3tb fusion set and after that everything worked perfect! I just lost my recovery partition but that's ok for me.
I know not everything went as expected but it's now working as planned (y)
Good luck
😉