I can already hear the gasps, but I have been running MSFS of a HDD since day one. Did some comparisons with a friend that has almost the same system but has three SSDs. ONLY measurable benefit was in initial load times and menu navigation. Once in the cockpit we could not find any differences.
The only deciding factor to truly consider is, longevity. If you decide to move your cache to a HDD you will be greatly increasing the R/W cycle on that HDD. An SSD does not care how often you read from it, only degrades with writes, and then not enough to to shorten it’s life less than yours. If you fly primarily in the same area then the sim is just reading data after the initial cache and it will do that long after MSFS is off the market.
An HDD on the other hand gets old fast with multiple R/W cycles. It doesn’t care if you are just reading or not. Every access degrades it. Lots of flying will result in the death of that HDD long before MSFS is ready for the heap. So ask yourself this. Is the life of that poor HDD worth the space gained on the SSD? Is there maybe some other ‘stuff’ that could be stored on the venerable old HDD that only needs occasional access, that would open up some elbow room on the SSD?