Our product took about a year to port, from getting the go-ahead to the eventual production switch. Running modernize / futurize was like 0.01% of the work.
You're right though, we're fighting our way out of technical debt and switching to python 3 was absolutely necessary. It's forced us to sort out a lot of sketchy string / bytes handling. We do, mercifully, have ~90% test coverage.
I don't remember us writing any new tests specifically for the python 3 port.
One issue with the test suite was that it made heavy use of a thing called django_any that isn't supported in python 3, so decided to replace it with Factory Boy. We have about 500 django models that needed new factories. Factory Boy works quite differently and it was a lot of work to make the factories behave similarly to the old ones where possible, and update most of our ~4000 tests for the new behaviour.
So that was one issue. It was tempting to just patch django_any, but we decided to tackle the technical debt instead.
You're right though, we're fighting our way out of technical debt and switching to python 3 was absolutely necessary. It's forced us to sort out a lot of sketchy string / bytes handling. We do, mercifully, have ~90% test coverage.