There’s no way of knowing, which is the whole problem with their model and why a lot of us self host things in the first place. Even if they super duper promise not to use the data, they could be lying. And if they are actually true to their word today, that could change tomorrow.
It is likely not worth your effort as whatever you come up with will likely result in discord deactivating your account for breaking their ToS, or them breaking their API forcing you to constantly play catch-up.
This is why open communication protocols are so important. Email is still as ubiquitous as it is because it’s a protocol, not an API.
I personally think it would be less overall effort to get your friends to switch to an open protocol like matrix, or XMPP than it would playing cat and mouse with proprietary APIs. But you do you, I wish you the best of luck!