I just posted my solution on r/vintagestory, so I'll just copy/paste it here:
I came up with a solution I call "Evil Screen Sharing". It's a bit unorthodox, but if you get OBS on steam or whatever you want, you can set it up with the virtual camera tool. By using this, you make the game capture act as a webcam instead. This actually "streams" at a higher quality than most other normal games, just by using OBS this way. If you want to stream sound too, I found that you can screenshare your second monitor at the lowest settings, and just enable the audio capture. This actually ended up working so well that I will probably be using "Evil Screenshare" with other stubborn games that refuse to capture correctly on discord. Hope this helps!