I’ll heartily endorse the idea of using a Mac mini rather than a NAS.
I do think there’s a middle ground between RAID and just an external drive backed up to some sort of cloud backup. On my Mac mini server I’m running pairs of external drives. One is the working drive while the other is a backup. Carbon Copy Cloner clones the working drive to the backup every night. That means if a drive dies I have local copies (in addition to cloud storage) I’ll never loose more than one day’s worth of data, but I don’t have the overhead and cost of a RAID system. I think this is a good compromise for data on the server, which usually doesn’t change much on a daily basis). Data that changes often is generally on the internal drive of my iMac and gets backed up to Time Machine on an hourly basis.