No. They cant decrypt your chats. They can however backdoor your device and see the pre or post delivery message. Its not hard for them to do. Technically or legally.
If they arent currently logging activity on your device then turning on self destruct messaging could mitigate their ability to spy on you. Unfortunately all your chat partners have to do it too.
They already know exactly who you are. People dont understand how pervasive online surveilance and profiling is. AI makes it simple to compile dossiers on everyone.