You can derive the cutpoints from baseline probabilities or model them directly with an induced-Dirichlet prior model. See my reply here with example code of the former including a prior predictive check.
See Ordinal Modeling for an excellent look at ordinal models with explanation, examples, and Stan code examples that include both deriving the cutpoints from baseline probabilities with Dirichlet prior over the probabilities or modeling cutpoints directly with induced-Dirichlet prior model.