if the data is mission critical have multiple backups for it, and test the restore process of said backups.
As for your raid vs software raid, just install truenas and throw it on there, use a LSI in IT mode and create a ZFS cluster. Don't over think it.
theres a reason you wont find many L2 "software" its extremely inefficient and kills processors. Switches use purpose built hardware to be able to hit millions of I/Os without using a lot of power because of this. If you are trying to use a generic x86 processor for this, well you will have a bad time.
https://vyos.io/