1.Q: Any benefits of just setting L1 cache (ram cache) only to cache SSDs and HDDs?
A: If you have enough ram for a large L1 cache, you can even cache NVMe drives effectively. I have a 30GB L1 on my 1TB NVMe boot drive which is less than half full, and get great hit rates if I consistently use the same software/game(s).If the size of your L1 cache is less than 10% of the total data to be cached on the target drive, your hitrates start to suffer (unless you really focus on only one piece of software).
2.Q: When you have prefetch last cache enabled it is my understanding that it will load the last cache to the level 1 cache. What does it do when the prefetch size is bigger than the l1 cache? Or does the prefetch information where id says Done (32gb/32gb) just mean data accessed?
A: It is pretty unlikely that a prefetch size larger than L1 cache size, unless you "lock cache content" and change the L1 cache size. But even the prefetch size is larger, PrimoCache will stop prefetching when L1 cache is full.