I built my own NAS, just Ubuntu+ZFS using an old PC I had and it works great. Don't know why people are so crazy about buying prebuilt NASes.
Its the maintenance and details that kill your NAS. You will probably rely on it for 10 years at least, so building your NAS means committing to maintaining it in the long term, you have to be sure this is something you will uphold. Also while setting up your NAS initially is definitely doable, there are a trillion little things to get right and make sure won't bite you later on (scrubble, etc). All these things comp…
A NAS is not a backup? It's hot storage.
I have a NAS but I backup my photos on AWS in S3 Deep Glacier. It costs me less than $1 a month for 500GB of pictures. That's a backup.