But when Youtube shares the key with me/my client the first time, is that also encrypted?
Here’s an explanation of what happens during the initial TLS handshake.
…if ISP automated the process of gathering keys and decrypting web traffic for a certain site with them for all users, would that work for them?
Not sure this is exactly what you’re asking, but there’s the concept of forward secrecy for defending recorded encrypted traffic from future key compromises.
in·se·cure (ĭn′sĭ-kyo͝or′) adj.
- Inadequately guarded or protected; unsafe: A shortage of military police made the air base insecure.
https://www.thefreedictionary.com/insecure
Unsecure
a. 1. Insecure.
I ran into something similar where the main Syncthing interface was limiting a folder to “Send Only” with that same “locked to read only” message. In my case I was able to work around it by going to the Web GUI and there I was able to set the folder in question to “Send & Receive”. I don’t know if it will apply to your situation, but you might give that a shot if you haven’t already.