I'm in China and have to use that piece of crap. So here's how I locked it down:

1. Root your phone with Magisk. There's no way around it.
2. Install Storage Isolation (https://play.google.com/store/apps/details?id=moe.shizuku.redirectstorage) and deny access to all folders.
3. Install ApOps (https://play.google.com/store/apps/details?id=rikka.appops) and set pretty much everything to deny or ignore (ignore means the app receives the information "permission granted", but no data is provided, in case some permissions are "mandatory"). If you intend to use wechat to exchange voice messages or make video calls/send photos, the "use microphone" and "use camera" functions would be required. In a similar fashion the location access if you intend to use the location sharing feature.
4. Be acutely aware that wechat is not encrypting messages, neither end to end nor in the server communicaton. Everything you say can (and probably will) be read and archived. Don't say anything confidential or critical there.

And yeah really, try to convince your ~~wife~~ girlfriend to use signal instead. Or hell, even whatsapp is miles ahead.

My wife is Chinese as well, so even after we leave here she'll be using wechat to stay in touch with family, no way around it, but using messengers more commonplace in other countries is definitely better. Personally I will move wechat to another phone once we're out. For now that's not feasible as it's too much integrated into every function of life here.