how to define a json type to a model field from django models

In the background JSONField actually is a TextField, so that output from sqlall is not a problem, that's the expected behavior.

Further, I recreated your model and it worked just fine, both when entering the value as a string and as a python dictionary, no character limitation whatsoever. So my best guess it that the issue is a completely unrelated field that does have a 15 chars limit.