I manage some QNAPs for a small company and I don’t recommend them. I’ve had many issues over the years. Their RAID solution is just MD/LVM. In my experience, Synology is more robust. I believe they use BTRFS, although I’m not sure for RAID5.
In general, FreeNAS is my recommendation. If you want to build your own, this post might be helpful for choosing well-supported components.