Well, I guess it's time to replace my otherwise perfectly-good Synology NAS on the double.
See the last couple of posts in the thread as well.
Sounds like you can get an RMA, but it's a slow process.
Instead, I'll shut down the NAS and buy a replacement from another company (QNAP probably) and transfer the data. The other options feel too risky.
(Synology 1515 specs: https://www.synology.com/en-us/products/DS1515+#spec)