My Steam library doesn't fit on the SSD. Caching the games that me and the kids actively play makes sense. Unfortunately, it doesn't look like I'll be able to have the OS and the cache on the same SSD this time.
Toliman already mentioned PrimoCache. I advocate it also to break away from proprietary solutions like ISRT. And also, I believe that ISRT will only work with an Intel SATA Controller, until, as some say, the right nexus of Intel hardware dovetails with NVME M.2 PCIE.
I'm now using the Beta version 3.0 of Primocache, and haven't discovered any bugs or problems. I also tried it with a dual-boot configuration for a single 960 Pro 1TB drive, by creating a caching volume of ~ 150 GB after the system-boot volumes. That works pretty well by itself. Didn't really have any problems with it.
You can, if you wish, have a two-tiered caching solution for the HDD spinner. You would use the NVME caching partition, and set up a RAM cache of . . . whatever size you thought was practical.
I had also done things that way, but discovered that my Macrium backup would eventually show disk errors on the cached HDD, requiring a CHKDSK repair. Once I stopped caching the HDD to RAM as well as to NVME M.2, the drive errors disappeared.
Reports tell me that the two-tiered caching works fine if you don't have the complication of a problem like my backup configuration.
While you would pay maybe $30 for a lifetime license of PrimoCache, you can use it with combinations of different controllers, RAID arrays, AHCI disks and so forth. It's a Swiss Army knife of caching options.
Currently, I no longer use the boot-system NVME for caching. Instead, I put in a 250GB 960 EVO using the second x8 PCIE slot. The entire EVO is configured as a caching drive for a large spinner. I may eventually move to a PCIE x8 card that allows for 2 or more NVME drives. But there's not hurry or need.