I used to … but with my recent Syno upgrade, I actually split into two separate Synos. One is primary with a RAID-5 (4 1), the second is purely backups (also 4 1). I do daily snapshots across to the backup unit, so even if I lose the entire primary unit, the secondary stays up and is immediately accessible.
I also do a nightly hyperbackup of the secondary array offsite to a family member’s house - and they cross replicate to my backup unit.
It’s about as sane a strategy as I can think of without getting too crazy.
I do keep a spare drive on the shelf - both the primary and secondary use the same size disk so I’m comfortable with just 1 at a time.