That 6 months is also a great opportunity to establish a test suite. It's dangerous to tell management that the code is bad. It's not dangerous to say that it needs tests, since that's an industry best practice and there are objectively no tests, and that you believe that a certain amount of your time needs to be devoted to paying back some of that testing debt. Then, when 6 months rolls around, you either have the confidence to do a major refactor/rewrite or leave and give the person who comes after you the test suite necessary to do the next step.

Plus, to my mind, there's no better way to start understanding a new codebase than to start with tests, either writing them or reading the existing suite.