The last time I checked OpenBSD couldn’t mount RAID1 if 1 drive was missing. Did this change in the meantime?
Last I heard, you couldn’t do nested softraid on OpenBSD. Is that ok now?
You could always do nested softraid. It’s just not brought up at boot time, in this example only the raid1 is configured at boot and the rest has to be configured/mounted manually.
So you are not able to boot from an nested softraid, that’s all.