Prediction: they will still see ads for the topics shared in writing because the surveillance being done is based on recording the inner speech of everyone with microwave imaging and machine learning.
Encryption is useless against advanced covert radars. Big Tech knows and benefits by lying by omission.
Pick two topics every time. Send one topic to your wife on Whatsapp. Write paper messages to your wife about the other topic and give it to her.
Track how often you see advertisements for topics in both channels. A significant difference in any one channel will be worth sharing.