Aurelius Ambrosius's main expense is the one legion he has remaining; while it does make him quite strong it's very expensive to maintain and might not be worth it if you're constantly in debt. However, as I recall Aurelius Ambrosius should have a large enough income to maintain it (make sure you're not extrapolating from the winter financial situation - holding income varies substantially between seasons).

Do you have Conclave enabled? IIRC you start with no vassal tax without it, so if that's the case you'll definitely want to change that law. Even if you do have it enabled raising taxes should still be an option.