Not OP, but having structure to a datastore is important. It decreases development and maintenance time and doesn't let vauge bugs fester for years. NoSQL is a premature optimization that many people make without understanding the downsides.
Completely agree. But in the above case, the structure of third party service API will always change and each change may require a migration. I worked on a similar problem in my last stint and used Postgres. Here I opted for predefined fields and handled the extra logic in application code. In the above case, I just wanted to know if there is a better way to solve the problem in Postgres.
I get a lot of mileage out of using real columns for the data with fixed structure, and Postgres JSONB columns for the parts of the data whose structure varies.