I ran a 'home-built' NAS for years... but finally bought a QNAP a few years ago and love it. (4x 5tb drives.. ~15tb of total storage)
QNAP (and Synology, Drobo.. etc) have built in utilities that sync with cloud storage. I have all of my photos and development work backed up to BackBlaze B2 once a week (my dev stuff gets backed up to other 'cloud' sources instantly). I pay ~$5 a month for BackBlaze since you pay a very low price on storage.. most of that money is in transaction fee's.
If you want to build a NAS you can use one of the opensource NAS projects (FreeNas) or its simple to do on Linux using mdadm.