The alternative to a Synology NAS isn't RPi. There are plenty of alternatives - QNAP, UGreen, a tower running TrueNAS - but a messy pile of overpriced unreliable SoCs attached to SATA hats isn't an alternative for a single device with multiple hard drive bays, consistent power and cooling, and easy management.
The alternative is anything not Synology that can do NAS with SATA SSD or NVMe storage. That’s it. Anything more than that is in a class of enterprise servers that deserve its own discussion over a simple DS1522+
This is nonsense. Both a horse and a pickup truck can be used to pull a wagon, but no one seriously considers one an alternative to the other.
What you are describing is a hobby item for enthusiasts who want to enjoy tinkering, setting something crazy up and constantly debugging it. That is very much NOT the market of people who buy Synology products.
Synology is not a JBOD RAID. It is an appliance that does many things (storage, services, web access, etc.) and can automatically keep itself up to date with no additional contact necessary. It can be hooked up to a UPS for resilience. If you have a problem there are support forums and online articles. It can be synced to other Synology devices at other sites. Etc., etc.
