>I have literally never seen any of them lock up when a drive fails
That's not really what happened. One of my buddies and I both have ds1815+ (both with 8 drives in RAID6), and both machines turn off when a drive fails, and both machines require you to physically disconnect the drives one by one until the machine boots, like "disconnect drive 1 of 8, try to boot. Reconnect 1, disconnect 2, try to boot" and so on.
What happened with my box is that 2 drives failed simultaneously. I managed to find the 2 dead drives, and tried to fix the RAID6. The rebuild failed, and the box locked me out. I wasn't locked out of the Synology log-in, I was locked out of the volume, and when the rebuild was over (and failed), this is what happened. Pic related is the log entries from the Synology.
Note that "shared folder [Media] was deleted" made me flip a bit of a shit, as the [Media] folder is the volume name that contained 11TB of my everything.
Throughout the process of drives being lost, the contents of the volume were not accessible. Basically, if a drive fails, you can't access your stuff until you finish fixing it, whatever the process might be. If something in the rebuilt fails, you're dependent on tech support to try and fix it. From the Synology forums, this seems to be the case with other units like mine.
Getting a second ds1815+ might be the answer. If one box has a dead drive, the files will still be accessible on the other box. If the rebuild fails, I can just delete the volume and make a new one, then transfer from the good box to the new volume.
The reliance on remote-access tech support is my main gripe. Even if it was complicated, I would much rather try and fix it myself.