Thanks for posting a good question and your valuable feedback!
Yes! Free Transport Layer Security (TLS) for Azure App Service has been one of the most highly requested features of the service since its inception. While this is still in preview and receiving feedback from the users, our product team is actively working on further enhancements.
For your question on Managed Certificate behind Traffic Manager - I'm checking on this internally and will get back to you soon.
Just to highlight, as mentioned in the blog App Service Managed Certificates (preview) and based on your requirement, "If you’re planning to do a live site migration with TXT record, need support for apex domains, or need a wildcard certificate, then use App Service Certificates or bring your own certificate." -