I live in Canada. My girlfriend is Chinese (also living in Canada), and while we are able to communicate via SMS, her mobile carrier isn't the best, and so there have often been issues for us with regular texting. She expressed a strong preference to use WeChat, at least as a backup option for when texting fails us. While I have some pretty significant reservations, it's not the hill I want to die on. So my question is: what can be done to use WeChat without compromising my whole phone? I'm okay with it if our conversations aren't private, but I'd like to know that I'm not giving unfettered access to all of my phone's systems and data to the CCP. What can be done to limit the reach of this ubiquitous app on my device?
Probably not the solution that you are looking for, but maybe try Signal? It's better than SMS, WeChat, WhatsApp and most other messengers. Unlike stuff like WhatsApp and SMS, Signal can even be used in China, because it has a built-in censorship circumvention system which uses special TLS proxies or can even be used over Tor.
This feature only exists, because the people behind Signal actually care about freedom and want to help their users. Unlike WeChat, Signal is not built by any government to spy on people, and unlike commercial messengers like WhatsApp and Facebook Messenger, it's not built by a corporation that wants to extract money from their users by selling their data. It's a non-profit organization, created by people who are dedicated to make the world more private and secure. Just keep that in mind when choosing a messaging app.