Aurora Store is 3rd party client for Google Play Store while MicroG is open source implementation of Google libraries so they are not quite comparable.
I'd say MicroG with Aurora Store is most private you can be if you really need to use Google services. You can use Aurora Store with anonymous Google Account that they provide you and MicroG is only sharing necessary data with Google servers.
Well yes, anyone can compile its own version of Signal and use it and it will work as long as there aren't some major changes to its communication protocol
Even though I don't use chromium-based web browsers because of its monopoly, it would be nice if we could integrate brave shields into ungoogled chromium so people wouldn't have to deal with all bloat and surprises from Brave
Data only needed for that specific service to function