Another option worth mentioning is Unraid.
It sits somewhere between a traditional NAS OS and a lightweight server platform. It uses a parity based array rather than RAID, which allows mixing different drive sizes and expanding storage incrementally.
It also has strong Docker and VM support, making it appealing if you want to run services alongside basic NAS duties. The tradeoff is that it is not free (one-time payment) and it is less appliance like than Synology, but for DIY builders who want flexibility and an active community, it is a solid alternative.