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