Hi, if a fusion drive works, it will only shorten the loading time. Don't expect any fluidity improvement.
One more thing, if you have a choice, spend the money in a computer instead of an iMac. You will have much better performance. Also note that currently, Nvidia GPUs handle X-plane way better than AMD GPUs.
There have been many threads about it, on this forum.
Reverting to the SSD, I personnaly have X-plane on a 240Gb SSD.The orthophoto I use often are on the SSD too, and the orthophoto I rarely use are on a 1Tb HD, I place shortcuts for them in X-plane 11\Custom scenery folder. This way I have best loading time almost every time I use X-plane.