Welcome to TrueNAS forums.
There is no such thing as a ZLOG, it’s SLOG or Log device. It is always helpful to use the proper terminology. ZFS does not have a write cache, the SLOG is not a write cache. All pools have a ZIL for synchronous writes, which is inside the data vDevs.
Further, if you have to ask about SLOG, Special Metadata vDev or L2ARC / Cache devices, you probably don’t need them. They can always be added after.
In general, L2ARC / Cache device(s) would only be 5 to 10 times your RAM size. So with 256GB of RAM, using 2 x 1.92TeraByte SSDs for L2ARC / Cache is a bit too much. This is because the pointers to the data in the L2ARC / Cache are in RAM. Too much L2ARC / Cache and too much RAM is used.
I highly suggest reading about ZFS and it’s pool configuration options. I don’t have a list of Resources or docs to read, but you can find some in the Resources section of this forum. (See left hand side, Resources’ Categories.)