Sync leans on Log.sys.mjs heavily and its output ends up in files - a quick look implies console.createInstance / ConsoleInstanceOptions might need tweaks to support that use-case. The prefs used by sync for logging will almost certainly need to change, so some migration will probably be necessary, MDN will need to be updated and about:sync will too.

This looks like mostly work that needs to take place in the Sync component so I'm moving it there, but please move it back if you think General is a better fit.