Not sure if this is the right answer but let me give it a try with the limited knowledge i have on this.
these are the resources that I found when I am looking up for tls enabling for my app service. please see if any of this works.
how to configure ciphersuite for app service
enforce tls vrsion
The client (browser) gives a list of cipher suites it can ...
If I am understanding your question correctly, what you are describing sounds like a perfect use case for SSM. You can store your RDS password as a secure string parameter encrypted by KMS, then reference it in you terraform file as a data source with decryption. With this method, you only have to reference your decrypted password, and never directly store ...