I found an answer to this. I think the following policy is not well-tuned, and may miss certain edge cases.
It did work for what I need - albeit not yet robustly for a 'do anything' sandbox. I'm posting it so it might help others.
If anyone develops a complete answer, I'd be glad to accept theirs
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "I can tag myself",
      "Effect": "Allow",
      "Action": "ec2:CreateTags",
      "Resource": "*",
      "Condition": {
        "StringEquals": {
          "aws:RequestTag/CreatedByUser": "${aws:username}"
        }
      }
    },
    {
      "Sid": "create and tag my roles",
      "Effect": "Allow",
      "Action": [
        "iam:CreateRole",
        "iam:TagRole"
      ],
      "Resource": "arn:aws:iam::*:role/*",
      "Condition": {
        "StringEquals": {
          "aws:RequestTag/CreatedByUser": "${aws:username}"
        }
      }
    },
    {
      "Sid": "AllowDescribeRoles",
      "Effect": "Allow",
      "Action": [
        "iam:ListRoles",
        "iam:ListRoleTags"
      ],
      "Resource": "*"
    },
    {
      "Sid": "AllowDescribeTaggedRoles",
      "Effect": "Allow",
      "Action": "iam:GetRole",
      "Resource": "*",
      "Condition": {
        "StringEquals": {
          "aws:RequestTag/CreatedByUser": "${aws:username}"
        }
      }
    },
    {
      "Sid": "AllowAttachRolePolicy",
      "Effect": "Allow",
      "Action": "iam:AttachRolePolicy",
      "Resource": "arn:aws:iam::*:role/*",
      "Condition": {
        "StringEquals": {
          "aws:RequestTag/CreatedByUser": "${aws:username}"
        }
      }
    },
    {
      "Sid": "ServiceLinkedRoles to my RDS",
      "Effect": "Allow",
      "Action": "iam:CreateServiceLinkedRole",
      "Resource": "arn:aws:iam::*:role/aws-service-role/rds.amazonaws.com/AWSServiceRoleForRDS",
      "Condition": {
        "StringLike": {
          "iam:AWSServiceName": "redshift.amazonaws.com"
        },
        "StringEquals": {
          "aws:RequestTag/CreatedByUser": "${aws:username}"
        }
      }
    },
    {
      "Sid": "ServiceLinkedRoles to my RDS",
      "Effect": "Allow",
      "Action": "iam:CreateServiceLinkedRole",
      "Resource": "arn:aws:iam::*:role/aws-service-role/rds.amazonaws.com/AWSServiceRoleForRDS",
      "Condition": {
        "StringLike": {
          "iam:AWSServiceName": "rds.amazonaws.com"
        },
        "StringEquals": {
          "aws:RequestTag/CreatedByUser": "${aws:username}"
        }
      }
    },
    {
      "Sid": "View and Change Own Policies",
      "Effect": "Allow",
      "Action": [
        "iam:ListPolicies",
        "iam:ListPolicyTags",
        "iam:GetPolicy",
        "iam:TagPolicy",
        "iam:CreatePolicy",
        "iam:CreatePolicyVersion",
        "iam:DeletePolicy",
        "iam:UpdateAssumeRolePolicy",
        "iam:AttachUserPolicy",
        "iam:AttachRolePolicy",
        "iam:ListAttachedRolePolicies",
        "iam:ListRolePolicies",
        "iam:GetRolePolicy",
        "access-analyzer:*Policy*"
      ],
      "Resource": "*",
      "Condition": {
        "StringEquals": {
          "aws:RequestTag/CreatedByUser": "${aws:username}"
        }
      }
    },
    {
      "Effect": "Allow",
      "Action": [
        "organizations:DescribeAccount",
        "organizations:DescribeOrganization"
      ],
      "Resource": "*",
      "Condition": {
        "StringLike": {
          "aws:PrincipalArn": "arn:aws:iam::*:user/${aws:username}"
        }
      }
    },
    {
      "Sid": "create my own RDS",
      "Effect": "Allow",
      "Action": [
        "rds:*",
        "rds:DescribeDBEngineVersions",
        "rds:DescribeOrderableDBInstanceOptions",
        "rds:CreateDBInstance",
        "rds:CreateDBCluster"
      ],
      "Resource": [
        "arn:aws:iam::*:role/AWSReservedSSO_*",
        "arn:aws:rds:*:*:db:*"
      ],
      "Condition": {
        "StringEquals": {
          "aws:RequestTag/CreatedByUser": "${aws:username}"
        }
      }
    },
    {
      "Sid": "Allow my S3",
      "Effect": "Allow",
      "Action": "s3:*",
      "Resource": "*",
      "Condition": {
        "StringEquals": {
          "aws:RequestTag/CreatedByUser": "${aws:username}"
        }
      }
    },
    {
      "Sid": "No STS on Prod",
      "Effect": "Allow",
      "Action": [
        "sts:AssumeRole",
        "sts:TagSession"
      ],
      "Resource": [
        "arn:aws:rds:*:*:db*",
        "arn:aws:s3:::*"
      ],
      "Condition": {
        "ForAnyValue:StringNotEquals": {
          "aws:RequestTag/environment": "prod"
        }
      }
    },
    {
      "Sid": "AllowUserTagSandboxedDatabaseAdministratorAccess",
      "Effect": "Allow",
      "Action": [
        "iam:PassRole",
        "rds:*"
      ],
      "Resource": [
        "arn:aws:iam::*:role/AWSReservedSSO_*",
        "arn:aws:rds:*:*:db:*",
        "arn:aws:rds:*:*:*:*"
      ]
    },
    {
      "Sid": "AllowUserTagSandboxedDatabaseAdministratorResources",
      "Effect": "Allow",
      "Action": [
        "rds:*",
        "secretsmanager:*",
        "kms:*",
        "cloudwatch:GetMetricStatistics",
        "cloudwatch:DescribeAlarms",
        "cloudwatch:ListMetrics",
        "logs:CreateLogGroup",
        "logs:CreateLogStream",
        "logs:DescribeLogStreams",
        "logs:PutLogEvents",
        "logs:PutMetricFilter",
        "logs:PutRetentionPolicy",
        "logs:TestMetricFilter",
        "sns:Publish",
        "sns:ListTopics",
        "sns:ListSubscriptionsByTopic"
      ],
      "Resource": "*",
      "Condition": {
        "StringEquals": {
          "aws:RequestTag/CreatedByUser": "${aws:username}"
        }
      }
    },
    {
      "Sid": "AllowUserTagSandboxedDescribeClusters",
      "Effect": "Allow",
      "Action": [
        "rds:DescribeDBInstances",
        "rds:DescribeDBClusters",
        "rds:DescribeGlobalClusters"
      ],
      "Resource": [
        "arn:aws:rds:*:*:db:*",
        "arn:aws:rds:*:*:cluster:*",
        "arn:aws:rds::*:global-cluster:*"
      ],
      "Condition": {
        "StringEquals": {
          "aws:RequestTag/CreatedByUser": "${aws:username}"
        }
      }
    },
    {
      "Sid": "RedshiftCreate",
      "Effect": "Allow",
      "Action": [
        "redshift-serverless:*",
        "redshift:*",
        "sqlworkbench:*"
      ],
      "Resource": [
        "arn:aws:redshift:*:*:*:*",
        "arn:aws:redshift-serverless:*:*:*:*",
        "arn:aws:sqlworkbench:*:*:*:*"
      ],
      "Condition": {
        "StringEquals": {
          "aws:RequestTag/CreatedByUser": "${aws:username}"
        }
      }
    },
    {
      "Sid": "semi-excessive list/get actions group",
      "Effect": "Allow",
      "Action": [
        "iam:List*",
        "iam:Get*",
        "iam:SimulatePrincipalPolicy",
        "iam:SimulateCustomPolicy"
      ],
      "Resource": "*"
    }
  ]
}