Sure, something like ZFS would be nice, but in a router not needed as you'd mount the fs rw only during update, wouldn't you? Logs are either on a separate, RAM based fs (tmpfs) or (preferably) sent to a remote logging host.

You might want to have a look at e.g. https://www.mimar.rs/blog/how-to-increase-openbsds-resilienc...

FFS can crash quite safely. The chance of lost data is incredibly small, especially with "softdep" out of the way. It just doesn't come back online as fast as a journaled file system will.