You can add a cache SSD to keep hot data to reduce access times, and why do you need that much of a throughput to begin with?

you can run ZFS without the Unraid disk array in unraid these days

Doesn't that get rid of one of the biggest benefit of Unraid where you can mix and match drives, just like in a Synology hybrid RAID?

I think this is just the tradeoff you need to make. I’m not aware of a solution where you can mix-and-match drives but also get the write performance of a traditional RAID array.

that is true, but you can make one fast pool using zfs and one slower one using unraids disk array, if you want to, or just use the zfs part as a cache for performance