AFAIK it’s not mandatory (but I’ll let the experts weigh in), but what would be the purpose of having super-high redundancy on the special VDEV and not in the pool? That would only make sense, if for some reason the special VDEV is prone to more wear and tear during the resilvering process, but (again AFAIK) that is not the case.
That makes sense. In this case though I’d recommend to be value-conscious (note that I’m not saying “thrifty”), in how you equip your system, so you can get to the stage of getting a backup system sooner.
They have been discussion on the forums about that and even in a recent TrueNAS Tech Talk. TLDR you do not NEED 64GB RAM, but ZFS will use most of the available RAM for its ARC read cache. I recommend you watch the section of that episode.
There is an ongoing discussion about L2ARC in another thread on this forum.
The nice thing about L2ARC is that you can try it out with a cheap SSD you have laying around, since it’s a read cache (as previously mentioned), so pool integrity is not jeopardized (only potentially the lifetime of your SSD in the long run). And you can always detach the cache from the pool. So zero investment to try it out.
The final answer will depend on the use-case for your system, which I didn’t see mentioned in this thread.