Ok i understand what you are saying, but... ts not a matter of "outlast" mate. It's a matter of complexity and creating a fusion drive using terminal seems kinda complicated to me and puts me into difficult paths regarding the integrity of my data. If it was just one huge SSD drive and onedrive making the backup then it would all be simple and clear i guess!
One large SSD would put you in the poorhouse
The Terminal commands are not too difficult, but if you are unfamiliar with the command line, it can be daunting.
The other way to go, if you don't mind ripping the Mac open again is an SSHD, such as
this. I am using this on a Mini 2014 in an OWC Thunderbolt bay as my boot drive. I use an older version of the 2.5inch 1TB SSHD in an older MBP 2008 and it is clearly faster than the standard HDD. For example, boot times with HDD on the MBP are probably 90-120 seconds, 30-45 with SSHD and 15-25 with SSD (I have tried all kinds of combinations on this old workhorse). Boot times on the Mini using this as an external Thunderbolt connected boot drive are in the 30 second range and apps launch with 1-2 bounces. Like Fusion, Seagate uses SSD cache to store commonly used files to speed up the things you use often.
As I understand, iMac supports either 2.5 or 3.5inch drives, this is a 2TB hybrid drive, it appears as a single drive to the Mac, yet gives you most of the benefits of Fusion without the complications. As a boot drive, it is close to the speed of SSD but offers the capacity in a simple way. However, if I am not mistaken, your iMac is SATA II with 3Gbps throughput, so the full potential of SSD and SSHD are not realized in this older Mac (same with my MBP), likely you would see very little difference in speed between SSD and SSHD.
Lots of options...