How to restrict which principals can appear in a role's trust policy

I'm trying to set up permissions so that my users can create roles and policies and use them to give AWS resources access to other AWS resources, but not use them to give humans access to AWS resources. For instance, I would like a developer to be able to create role and attach policies giving access to an RDS instance and certain SSM parameters, and then attach that role to an EC2 instance. However I do not want that developer to be able to give, say, his college roommate, access to that RDS instance and SSM.

We have an AWS Organization with multiple accounts, mostly split along cost allocation boundaries, and use AWS SSO to control access to them.

My first attempt at this was to create an SSO permission set with permissions to create roles and policies, and then an SCP to limit who can use the roles.

So I created a "Dev" permission set with the PowerUser and IAMReadOnly AWS managed policies, and the following inline policy:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "CreateAndManageDevRolesAndPolicies",
            "Effect": "Allow",
            "Action": [
                "iam:CreateRole",
                "iam:DeleteRole",
                "iam:UpdateRole",
                "iam:PutRolePolicy",
                "iam:AttachRolePolicy",
                "iam:UpdateAssumeRolePolicy",
                "iam:DeleteRolePolicy",
                "iam:DetachRolePolicy",
                "iam:TagRole",
                "iam:UntagRole",
                "iam:CreatePolicy",
                "iam:CreatePolicyVersion",
                "iam:SetDefaultPolicyVersion",
                "iam:DeletePolicy",
                "iam:DeletePolicyVersion",
                "iam:TagPolicy",
                "iam:UntagPolicy"
            ],
            "Resource": [
                "arn:aws:iam::*:role/Dev*",
                "arn:aws:iam::*:policy/Dev*"
            ],
            "Condition": {
                "StringEquals": {
                    "iam:PermissionsBoundary": [
                        "arn:aws:iam::aws:policy/PowerUserAccess"
                    ]
                }
            }
        }
    ]
}

and then this SCP:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "DenyUseOfDevRolesFromExternalAccounts",
      "Effect": "Deny",
      "Action": [
        "sts:AssumeRole"
      ],
      "Resource": [
        "arn:aws:iam::*:role/Dev*"
      ]
    }
  ]
}

the idea here was to test that this SCP was properly denying access to all Dev roles, and then add in some conditions to loosen it up to the access I want.

However, what this SCP actually does is deny access to all of my organization's users (even if they are listed in a Dev role's trust policy), and allow access by all other AWS accounts (as long as they are listed in a Dev role's trust policy). Essentially external-only access to the Dev roles, the exact opposite of what I actually want.

Is there any way to achieve what I'm trying to do?