RAID setup require two interfaces (same) and drives which are running at the same I/O speed. Thats not possible in this series! Here you have a PCIe/NVMe interface and a SATA interface.
Fusion Drives are a compromise! Leveraging the SSD as a cache to the slower HDD. Thats why the SSD’s Apple uses are on the small size. Using slightly larger SSD’s can be helpful at some point the benefits drops like a stone and the costs become wasteful!
The best you can do here is setup a dual independent drive config! Get a 500 GB or larger PCIe/NVMe blade SSD set it up as your boot drive hosting your apps. Leave the rest of the drive empty so the OS can leverage it for virtual RAM, caching & paging. If your apps use scratch space make sure its setup to use the SSD. Use the HDD as your data drive. If you need to upgrade it consider doing with a SSHD drive. Going with a 2.5” SSD doesn’t offer any true benefit in this config given the cost.
The Ultimate Guide to Apple’s Proprietary SSDs
Seagate FireCuda 2.5” SSHD