Set Iam Policy using Resource Manager Python V3 GCP API doesn't have a policy field?

Question

I am trying to add service accounts to a certain role in a project in GCP. In order to do so, I get the IAM policy and then add the accounts desired, but am unable to get set_iam_policy to work correctly. I add the projects_to_add to the member list for the role I want to add service accounts to, but the GCP Docs are confusing and I'm not able to add the roles_dict I updated to set_iam_policy. Does anyone know how I should do this?

client = resourcemanager_v3.ProjectsClient(credentials=credentials)
    req = iam_policy_pb2.GetIamPolicyRequest(
        resource="projects/mse-sandbox-registry-anushka"
    )
    response = client.get_iam_policy(request=req)
    roles_dict = MessageToDict(response)

    projects_to_add = []
    projects_to_remove = []
    for proj in proj_numbers:
        account = (
            "serviceAccount:service-"
            + proj
            + "@serverless-robot-prod.iam.gserviceaccount.com"
        )
        if proj not in storage_obj_viewer_list:
            projects_to_add.append(account)

    for dict in roles_dict["bindings"]:
        if (
            dict["role"] == "roles/storage.objectViewer"
        ):  # add the projects into the dictionary
            members_list = dict["members"]
            members_list.append(projects_to_add)
            dict["members"] = members_list

    req = iam_policy_pb2.SetIamPolicyRequest(
        resource="mse-sandbox-registry-anushka", Policy=roles_dict
    )
    response = client.set_iam_policy(
        request=req,
    )```


I tried various forms, including passing in resource="projects/<my-project-name>", policy=roles_dict, but nothing seems to work.