Serving VMs, means serving block storage. And that implies a SLOG.
For block storage running off rust, you’d want pairs of mirrors… so take your biggest and fastest rust drives… and use those.
Grab a pair of enterprise SSDs, the ones with the best synchronous IOPs and use them as a mirrored SLOG. Although you don’t need to mirror the SLOG… it comes down to your tolerance for missed writes… the mirror prevents losing transactions in the event that you have an unexpected restart etc, and that’s when you find that one of your SLOG devices failed/fails.
You’ll need a small/cheap SSD for the boot too. And you may want to mirror those too. Again, not necessary, but means that if one fails, you can easily replace it without bringing down your system, or easily recover.
And then you can decide if you want L2ARC. The L2ARC has the benefit of being persistent, so can help even if you have plenty of ARC, eg, when you restart to apply a system update.