With any large legacy Python codebase, that's usually a month-long undertaking for one engineer, if not longer!

Python by design encourages duck typing, which means that you'll have plenty of places that simply take multiple different types, and just slapping a union is usually non-trival and not very beneficial either (eg. you'd be randomly patching things with fakes and mocks in tests).

If you think it's a single day job, I am sure my company (and plenty others) would pay you gladly your single daily rate to get our codebase migrated in a day :-D