"An lvm(8) cache Logical Volume (LV) uses a small and fast LV to improve the performance of a large and slow LV."

Yeah, readyboost is(was?) a faster LV.. but not really. It was transparent to the user, and only used when it was actually needed. Seemed to eliminate your main complaint of fragmenting partitions in order to corral Fuzedrive to working like you wanted.

I wonder how/if an SSD in USB (as flash drive) would benefit a 10TB raid for games etc.

I'm just failing to see a usecase where fuzedrive is worth the resources/hassle. It always seems better just to use it as a scratch disk for rendering or install current games to it instead of letting it cache pointless stuff like media.