I think you would not need to change all endpoints to Post. Keep them like that and test it. If you get this same error then you need to add a result key on the get all endpoint. I think that should be response_data and outputs.
But you said this is a Post one and Custom actions will be good for that!
Yes, you are right, it is a post endpoint only. I can try custom actions - do you know how I can get started? Also, do i need a collection to work with custom actions? How do I get a collection if Adalo doesn’t allow me to?