German/Australian tax?

Hi, i’m a german citizen living since 2 years on different working visas.
Now I’m not sure what i shuld choose as country of residence.
And if i choose Australia, can i change it later to Germany in case i have to go back one day?
And as far as i understood so far i have todo all the taxes by myself at the end of the year at the tax declaration?

Sorry for so many questions, i’m trying to get into it but found quite a few contradictory answers to the questions.

Thanks for any help

Kind Regards

Country of tax residence is not something that is up to you to choose. Except in some special cases it is generally the place where you spend more than half the nights in the year. The websites of the Germany and Australia tax offices will surely have something to explain their rules. Eg

Yes, you do have to figure out and pay your own taxes on investment income. This is not something that a broker can do for you. Some people find they need the help of an accountant if investment income is significant or if tax returns have to be filed in more than one country,

Just as an aside - US citizens who live abroad have a particularly hard time. They are assessed for US taxes and tax in their country of residence even if they never set a foot in the US. The requirement under FATCA to report on accounts of US citizens is one reason why many brokers (Trading 212 included) ban US citizens from using their services.