I swear I looked everywhere, and disabled almost everything (insertion of matching brackets etc), but when I type an opening brace and hit enter, it auto-indents the next li
The right thing to do here is to disable smart insert mode completely (Edit > Smart Insert Mode), or configure the Smart Insert Mode (Windows > Preferences > Java > Editor > Typing).
White Fang gave you the correct answer, but I will give you a guide how to find options that you need:
The search in settings is extremely useful. When you type "indent", it would filter the irrelevant items in the tree.