We've come pretty far with something like this as a base policy. Maybe you can edit it to your own needs.

But as I said, it is not possible to make sure user can create buckets but not object lock buckets at the same time. But if user cannot create buckets (and cannot change OL-Mode for existing ones), you can control things like this.

In this example a policy that allows GOVERNANCE and Mode (and no object lock) but not COMPLIANCE.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": "s3:*",
      "Condition": {
        "StringNotEqualsIgnoreCase": {
          "s3:object-lock-mode": "COMPLIANCE"
        }
      },
      "Resource": [
        "arn:aws:s3:::bucket",
        "arn:aws:s3:::bucket/*"
      ],
      "Effect": "Allow",
      "Sid": "policy-gen"
    },
    {
      "Action": [
        "s3:BypassGovernanceRetention"
      ],
      "Resource": [
        "arn:aws:s3:::bucket",
        "arn:aws:s3:::bucket/*"
      ],
      "Effect": "Deny",
      "Sid": "policy-gen"
    },
    {
      "Sid": "VisualEditor0",
      "Effect": "Allow",
      "Action": [
        "s3:ListBucket",
        "s3:ListBucketMultipartUploads",
        "s3:ListMultipartUploadParts",
        "s3:GetObject",
        "s3:GetObjectVersion",
        "s3:GetObjectLegalHold",
        "s3:GetObjectRetention",
        "s3:ListBucketVersions",
        "s3:GetBucketVersioning",
        "s3:GetBucketObjectLockConfiguration",
        "s3:GetLifecycleConfiguration",
        "s3:AbortMultipartUpload",
        "s3:DeleteObject",
        "s3:DeleteObjectVersion",
        "s3:PutObject",
        "s3:PutObjectRetention",
        "s3:PutObjectLegalHold",
        "s3:GetBucketLocation"
      ],
      "Resource": [
        "arn:aws:s3:::bucket",
        "arn:aws:s3:::bucket/*"
      ]
    },
    {
      "Sid": "VisualEditor1",
      "Effect": "Allow",
      "Action": [
        "s3:ListAllMyBuckets"
      ],
      "Resource": "*"
    }
  ]
}