I disagree with the idea of not having a service layer at all, because in practice, changing your data model in some way is actually pretty common, even if the API remains the same so the data coming in/out isn't changing. I've had to split models into multiple parts to add audit history on certain fields, create notifications on object creation or modification (possible but hard to follow and a visibility problem when using signals), change the normalisation on fields, etc. etc. and at this point, turning to a service layer is much more appropriate than putting methods on the model since often the data is split across multiple database tables. I also would advocate this in projects where it's Django + DRF since you generally want exactly same logic to be run through whether submitting via a form or submitting via an API.

I've also found that it's good practice to impose boundaries between apps so that they are not strongly coupled together but communicate through an interface. That way, if you need to split an app out into its own project at some point, the migration is significantly easier from a code perspective, since the implementation of the interface function is all that needs changing on the consumer side.