I vaguely remember that you can copy the Phoenix source file /priv/templates/phx.gen.schema/schema.ex to your own project and edit the file and this version will be used by the generators.
I couldn’t find docs, but I dug a bit deeper into the source and found that the helper module for the Phoenix generator tasks includes this function phoenix/lib/mix/phoenix.ex at v1.5.11 · phoenixframework/phoenix · GitHub
Which suggests you can copy the schema.ex, make your edit and future generators will use it. It’s worth a try.
