The full strength of the SOP applies by default. CORS is an insecurity feature that relaxes the SOP. Unless you need to relax the SOP, you shouldn't be enabling CORS, meaning you shouldn't be sending an Access-Control-Allow-Origin header at all.
If your front-end at www.example.com makes calls to api.example.com, then it's simple enough to just add www.example.com to CORS.
So I do local dev on https://local.qa.yourappnamehere.com
> no synchronized password manager is safe
Care to elaborate? I'd agree that the security/availability tradeoff is different, but "not safe" is as nonsensical a blanket statement as "all/only offline/paper-based/... password managers are safe".