NAS for backup / file sharing

Hi there ! Just had a SSD failure on my computer and realized I had to do something in order to keep my datas from disappearing. I’m seriously considering buying a NAS but I think something like a Synology is overkill (and overpriced) for my needs. Can you give me some advice on what to buy considering what I want to do : - backup of my important files (pictures / documents) - file sharing (for movies, but I don’t need transcoding as I don’t plan to use Plex, I play my movies from an Apple TV via smb) - securing my datas with a raid 5 array, so I guess 4 bays for HDD at least. My internet box has a NAS function but can only fits 2.5" inch HDD (4 of them). Thank you for any advice or ideas.