Sometime ago I found an alternative "OS" for my DNS-320L called Alt-f [0]. Granted its UI is very old-school but it has worked flawlessly. There has been no release since 2022 but at least it's more recent than the last official firmware (and I assume is not impacted by the vulnerability).

IIRC I didn't even had to format or move the data I had on it before installing.

[0] https://sourceforge.net/projects/alt-f/