If your boat is 15 m or more and it will be operating on the open sea (not inshore) you will need a Registration
at the “Seeschiffsregister”: Willkommen bei der deutschen Flagge! â€” Startseite
If the boat is smaller, a flag certificate will suffice.
As far as I know there is no annual tax. VAT will be due when the boat enters European Waters.
You will need documentation
(sale contract), documentation
of giving up the other flag, your passport and you need to choose a home port.
You will also need a Measurement Certificate which you get from the Bundesamt für Seeschifffahrt und Hydrographie
As your boat is already registered in France
the CE Certificate should not be a problem.
Let me know how it worked out for you, I’m planning the same process.