I think state of the art in this space is WHIP/WHEP, which is what this project is implementing. I'm guessing 100k+ clients would be pretty difficult for Broadcast Box (out of the box), however Cloudflare has a beta stream service that's has a tagline of "Sub-second latency live streaming (using WHIP) and playback (using WHEP) to unlimited concurrent viewers." https://developers.cloudflare.com/stream/webrtc-beta/