Need suggestions to a list of users which have project permission with one API

We need to retrieve all those users list which have permission for a project. We are able to fetch assignable users but that was not worked properly as per our functionality. It will be good for us If you will provide details.

We can fetch users according to project by following below steps mentioned in url :

But for this we need to HIT 3 API Request and which seems not too feasible. To hit these API request user need Administrative permission. When I trying to HIT same APIS with External Jira User I will get Below Error :

“errorMessages”: [
“You cannot edit the configuration of this project.”
“errors”: {}