Sure, usually the campaign checks for locked items and unlocks them in the process. Doing them previously with some cheatcode can generate a loop that prevents you from moving forward Or like when you enter a locked area and no melody plays or no enemies spawn.. But this may be a bug, and Monolith needs to fix it. Since, although I still don't know, if there is a New Game + to start over, with all the classes unlocked from your previous game, there shouldn't be any bug that prevents you from advancing, and the cheatcodes that unlock classes should behave the same as a New Game + .
This cheat code will complete The Ouroboros Tree of Souls 100%