What happens if the NAS dies though? What does recovery look like?

Is it possible to recover the data from the drives without Synology’s OS? If so what is that process and how difficult is it to do correctly?

I know that with ZFS, recovery is independent of vendor OS and/or hardware, so if the hardware dies you can just throw the drives into any COTS system with enough ports, but I’m genuinely unsure if that is the case for Synology or not.

ZFS, btrfs, and other software RAID solutions can use mixed drives w/o much issue as long as you make sure that the capacities match or that you set the array up with the smallest disk size in mind.

Do not use hardware raid controllers. They provide no meaningful performance benefit over software raid and make data recovery much more difficultm(if not impossible) in the event of hardware failure.