If you are serious with this relationship (or you expect to still have Chinese partners in the future), I strongly recommend you buy a separate device for all the Chinese spywares required to maintain communication with your partner(s). At some point you will have to enter China, and it is best that you take only this device with you into it then.

use https://f-droid.org/packages/com.oasisfeng.island.fdroid/ (work profile) to isolate. and use a pay as you go number to register (rather than you commonly used one). do not grant any permission. set background restriction to strict. force stop it after every use. i think that would be enough

Why don't you use a better messaging app? If not matrix, then signal? Even telegram is better compared to both wechat and sms.

Weird perspective for someone using Google services lmao.

And you're not concerned about Alphabet, Microsoft, Meta or even the NSA?

Just tell her there's email. If she insists on WeChat... Maybe she's... err...

I mean, a messaging app isn't suited for a back up. Just throw her phone into the toilet, and many such apps are gone forever.