But since I'm stubborn Finn (and proud of it ;-) ), I would like still suggest new approach:
browser add-on/plugin for chome(chromium) and firefox browser(s) which uses fsecure server for storing sensitive information and browser-plugin code for handling typing the passwords.
I'm quite sure that data can be stored anonymously so that account is 'owned' by just string that belongs to browser-plugin. That should be, if not impossible, alleast extremely hard to track to actual user. Virtually as hard as tracing communication between devices that are syncing passwords with Keys? However, I might be wrong with this assumption, so correct me if I'm totally worng with this.
Anyway, if you look up market development, it seems that Android is number one platform, Windows as well, but the last one is declining. However, no matter which platform people are using on their traditional computers, they all are using browser, like Firefox, Chrome. Browser add-on/plugin could handle all platforms. (Or then not, I have never created browser add-ons...)