I would think about server replacement. Docs show that server tops out at 32GB of RAM. You are also limited in how many drives it can take. Link at bottom.
How many available hard drive ports are there? If you can add two more 6TB or larger HD, You could add another VDEV of a mirror pair for a bit more performance. You would end up with two mirror vdevs of 2 drives wide. 32 GB of RAM would be a good idea. I would stay away from special VDEVs, except L2ARC or SLOG (if you have sync writes). Both are covered in basics section.
One option could be contacting TrueNAS Enterprise and go over the Mini line or Enterprise lines for replacement servers to see what prices and recommendations they would have.
Linking Basics since I don’t know your knowledge level.
BASICS
iX Systems pool layout whitepaper
Special VDEV (sVDEV) Planning, Sizing, and Considerations
SLOG is not a write cache. You only need it for sync writes. So databases, Block storage (iSCSI, zvols for VMs), and NFS.