Hi @DaVinci
The rules around this are a bit complicated so I will try my best to answer it clearly.
In general, people have to apply for the correct visa (in your case visa for family reunification) before entering Germany.
However, people with a resident permit from another Schengen country (such as France) can apply for a residence permit directly here in Germany.
If you apply in Germany, the authorities may first reply that you need to leave the country and return with the correct visa.
If this happens, you should refer to this legal text: paragraph 39, section 6 of the Aufenthaltsverordnung:
https://***.gesetze-im-internet.de/aufenthv/__39.html
The rule states that a resident permit can be issued in a case such as yours. It does not state that it must be issued so the authorities still have some discretion. However, here it also important to note that the type of permit you are applying for, as the spouse of a German citizen, is a permit they must issue and so I would argue that they also must allow you to apply for it in Germany.
Best,
Éanna