Another nice place to brunch in Friedrichshain is Kurhaus Korsakow in the Simon Dach Kiez. It kinda looks like a hunter house, but don’t worry if you don’t like meat, it’s vegetarian friendly. For about 10€ you can help yourself at the big buffet with all brunch classicals, including lots of salads of all kinds, the staff always make sure everything is full and fresh.
For a few € more, you can get also a formula that includes prosecco or hot drink. The service is nice but can be long (fot the drinks that is).