The correct way to do so is to set issue_opaque_access_tokens: false in migrations as you have already done so. With that said, I can confirm that modifying the issue_opaque_access_tokens is the correct way to change the Fixed Length of Access Token & Authorization Code in your tenant settings. Everything looks good here.
However, I noticed that your picture_url and support_email properties in the tenant object have more than one value and are minimized.
Generally, the picture_url and support_email properties can only contain one value and are of type string. In this case, you will have to adjust your code to follow the expected syntax.
Both the picture_url and support_email properties have only contain one value. Even after adding that flag in YAML, I still see the fixed-length flag turned on. Where can I find the import logs that you asked?.
The import logs should be on the command line interface you used to call the import command. Please share the complete logs with me to see what is happening.