RAID setup advice

I have a server with 6 x 1TB drives in it. This is used as a secondary backup server and so read/write performance is not paramount. I am intending to move it from the current Linux-based RAID6 setup to OpenBSD. The architecture is amd64.

My primary requirement is redundancy in the case of disk failure for the data partition. A secondary requirement is to maximise space (minimum 3TB).

Given that booting is only supported from RAID1 and crypto, how should I arrange the RAID array? One option is to dedicate one drive, to boot from, for the OS and make the remaining 5 drives a RAID5 array. Is there a better way?