For L2ARC you want to have at least 64GB of RAM.
I believe you do not need a SLOG since you don’t seem to use sync writes.
A backup pool on the same system is kinda wasted, but that’s my opinion.
Suggested readings are Introduction to ZFS | TrueNAS Community, iX's ZFS Pool Layout White Paper and Some insights into SLOG/ZIL with ZFS on FreeNAS | TrueNAS Community.