Lots of excellent advice here and I see no reason to weigh in, but I will anyway.
What I want is a Time Capsule replacement. TC, as you all know, is not RAID, but many (most?) of us are using one as part of a comprehensive backup scheme.
Elsewhere I’ve seen some enthusiasm for using a Raspberry Pi as a TC replacement. Personally I think this is a terrible idea.
The great thing about TC: it “just works”, and it’s quiet.