Maintaining tests for API's can become a heavy task depending on how many API's and routes you have to maintain and how often things are added.
One could also think of offshoring the testing to a dedicated company. I gave it a try and received offers which calculated a fee for each route to test.
Unfortunately, none of the companies I contacted bothered going into details what kind of tests they will deliver.

I use the API first or design first principle to outline the requirements of the API under development and do this by defining it with OpenApi specifications.
I wrote an article some time back of the tools I use to write OpenApi definitions.