Our Low-Light HD USB Camera does the H264 compression onboard, and we unfortunately don’t have direct control over the bitrate. From previous investigation the firmware tries to maintain the bitrate (presumably while increasing quality) until very small frame sizes and low frame rates, and even then it’s still in the Mbps range.
You’ll likely need either a different camera (with fine-grained bitrate control and/or more efficient compression (H265?)) and/or a custom streaming system.
It’s worth noting that compression only goes so far, and at some point you can’t send information without significant losses of data (H264 is already quite lossy compression).