Privacy

I will find another app that's FOSS and does more or less the same thing. Otherwise I learn how to do without it

This is a very difficult question. In general, stuff such as microg tries to provide a layer that "translates" calls from google services to something else and tries to give a response back. However, as you can imagine, not all calls are there because either: they are not documented, or people at microg didn't have the time/manpower to do them, or it requires some sort of authentication method(s) and/or keys that google holds of. For situations like this, most of the time there isn't really a good solution for this. You either:

• try to use a web version of whatever you need to use
• check if there is some sort of wrapper for that service that you can use (I.e. something like fritter from back in the day, to access "indirectly" to twitter)
• have a separate device that have the google services as expected

I am aware this does not answer your question, but graphene os for instance does have the full (sandboxed) google services available for install (even this on certain edge cases can give issues, but its rare). Other Roms such as divest and calyx use microg instead. Either approach have good and bad things, but as far as comparability goes, sandboxed services is always better.