does anyone know how to do this, I find for when I'm going back to beat some records mixer buffers, starts late, and is just all around a not so good platform for speedrunning

Mixer is a widely used platform, so I highly doubt that it’s a problem in their end unless they’re experiencing some downtime. If you’re looking for alternatives, you can try to stream on Twitch instead through the app. But I would suggest hard wiring your XBox instead of using it over WiFi if it’s doing that as that might be causing a lot of your issues with drops.