Wonderful to hear from you again.
Do they require you to actually use the textbooks or just be able to pass the exam? If the latter, I would look at the topics covered in the textbook and, if possible, find living books to teach those topics. Any topics that aren't covered in a living book, you can use the textbook. It's okay to use a textbook sometimes; living books are just more engaging. Also, adding a good literature book to read aloud to your child will enhance the education. There's many good German books, like Heidi, and I'm sure you know about many more than I do.
I know you said there are not many families homeschooling in Austria, but if possible, find a homeschool group or even just one other family that homeschools. It will really help to see what books and resources the other families are using, even if they are not using a CM style.
I'm excited for you. It's wonderful to hear how homeschooling is growing around the world.