The idea of SSD only storage is great. Programs and games become larger. It will significantly lower load times. Especially, battle/level/map load times in games. Though split SSD storage is not that good. For the same reason of larger files. I was using 240 SSD and couldn't fit all my games into one drive.
So i see no point in boot drive smaller than 500GB.
First clarifications:
1. SATA is a protocol, not form factor. SATA SSDs can be in M.2 (any length), 2.5" and 3.5" form factors
2. M.2 is a connector type. the SSD for M.2 can use either NVMe protocol or SATA protocol. NVMe SSDs are faster than SATA ones. Bit in real life usage, the advantage is very small. for example windows boot time is 4-6 seconds with any of them.
so instead of split storage, get a single 1TB ssd like this one
https://pcpartpicker.com/product/RKhj4D/crucial-mx300-11tb-m2-2280-solid-state-drive-ct1050mx300ssd4 and save the 100$ on the 960 EVO
if you insist on NVMe, a 1TB 960 EVO will cost just a bit more than 1TB SSD + 250GB 960 EVO. ~440$