Hi,

export plate can be valid from 15 days to 365 days. TUV has to be valid for the whole time you choose.

Tax: it is no export tax. Its just normal vehicle tax everyone pays. Shouldn't be too much. Depends on engine displacement and emmisions. My 2009 Yamaha XT660Z is about 50€ a year. With access to a german bank account you just need to state that bank account (with collection authorisation if it is not yours) when you register that vehicle.
If not, you get a document, which states the ammount of the tax and you need to go to customs (which might be at a different place) and pay it there, then back to the registration.

Just read that you life in France: You might be able to use your frensh bank account now.

insurance: the insurance company will give you a code/number (called "eVB") and they tranfer your data to the registration office and you just need to tell them this eVB. I have no clue if this would be possible with a frensh company. BUT... with the german liability insurance you get a "green card" on which all the countries are listed, where this insurance is valid. This is normally all of Europe and even often some countries beyond. For example my liability ins. for my motorcycle is valid in Russia and Morocco too. I don't need to buy extra insurance there . Which countries are covered might vary from one insurance company to another.
The ADAC (like AAA or AA) will sell insurance for export plates. They say for 29 days it will cost 55€. It looks like you can walk in one of their office and get the insurance there. We have one in town and I can ask if you like.

When you go to the registration office, they might want to see your bike there for the registration (checking VIN). It's not allowed to drive the bike without any number plate on it. If you can't put it on a trailer, there are some possibilities. the easiest way would be, if the former owner gives you the motorcycle with the plate and registered to him and you just ride it to the registration office (you need to demount it plate and take it into the office to remove a stamp on it). But the former owner need to trust you for that. Often he will sell you the bike without number plate.
Buying from a dealer, he might give you a "red numberplate" which you can use to ride to the registration office.
Only if you buy a new motorcycle the registration office might not need to see the bike.

hope that helps
Martin