SLOG VS CACHE VS WAT - new build

I currently have a RAID 6 x 6 drives x 6tb for a total of yesiknowishouldvedoneraid5 tb. They are all HDDs.

I’m upgrading everything but the drives, I have a high budget of about $1200 and can throw in a bunch of RAM (doing 128 as I’m doing a bunch of VMs) and even a few NVMEs. I’m centering it around this motherboard with a 9900x Ryzen and 128gb RAM. I have lots of PCIe so I can put a bunch of NVME if I want.

My question is, for this setup (36TB HDD) - what specific NVME helper drives should I have for read/write help and what layout. Keep in mind I can’t rebuild the pool at this point (I don’t have capacity to back it all up at present).

SLOG - I get mirrored, but how big and what does this get me?

Special VDEV - do I want this? How big and does this need to be mirrored?

Cache (L2ARC,right?) - how big? how good? I’m guessing no mirror?

Are there options I’m missing?