@Dan[br]
Hi Dan, has anything changed about the situation since 2019?
I have a Mini Late 2014 with 2.25 TB Fusion Drive (2 TB HDD and 250 GB SSD).
What about the Samsung 970 Plus 2TB - now 3 years later? Does it work with the KALEA-INFORMATIQUE M.2 SSD adapter without any problems? Is it recognised as a boot drive? I would then just swap the 250 GB SSD for the 970 Plus SSD and use the HDD as a data drive, i.e. split up the Fusion Drive.