Hi there,
i have one 12 TB disk (7200RPM) for file storage. It performs random reads/writes. I know that writes cause read-modify-write overhead.
Will it make sense to use small SSD as L2ARC ( secondarycache)?
Want to avoid performance bottleneck and other problems.
Also would be great to get ZFS pool configuration proposals 
This is not correct. Read-modify-write essentially never happens with ZFS unless you’ve set ashift lower than the actual physical sector boundaries on the underlying media.
I’m not sure how many configurations I can offer for a single-disk pool… but you probably don’t need L2ARC or SLOG for the most part. On the other hand, an L2ARC configured with secondarycache=metadata might be useful, particularly during cold boots before the ARC is hot.