See the resource section re: sVDEVs and learn what they do and don’t help with. As @etorix mentioned, if you lose your sVDEV, you lose the pool, so the sVDEV should be as redundant as the rest of the pool (my sVDEV is a quad-mirror for a Z3 pool).
To get a consistent 1GB/s file transfer with standard 7200RPM HDDs likely requires four Z3 VDEVs in your pool. That’s usually a lot of HDDs.
NAS pools / datasets meant for fast I/O are frequently set up as a bunch of parallel mirrors. You lose 50%+ of pool capacity to parity but gain IOPS.