There are lots of simple things that are normally easier to do in the web framework that are suddenly easier to do in the database (with the side effect that you can do DB optimizations much easier).
But the other consideration is that you likely need to do a lot with a reverse-proxy like traefik to have much control of what you are really exposing to the outside world. PostgREST is not Spring, it doesn't have explicit control over every little thing so you're likely to need something in front of it. Anyway, point is that having a simple Flask server with a few endpoints running wouldn't complicate the architecture very much b/c you are better off with something in front of it doing routing already (and ssl termination, etc).
But the other consideration is that you likely need to do a lot with a reverse-proxy like traefik to have much control of what you are really exposing to the outside world. PostgREST is not Spring, it doesn't have explicit control over every little thing so you're likely to need something in front of it. Anyway, point is that having a simple Flask server with a few endpoints running wouldn't complicate the architecture very much b/c you are better off with something in front of it doing routing already (and ssl termination, etc).