Well i would go for owncast and obs, plus a jitsi for remote speakers.
When you plan something like that you want to test it on your chosen location/network, have a dedicated network for all your streaming so the wifi clients arriving on site will not disturb or eat your whole uplink. You start well before the actual time and have a backup streaming solution ready to go, or directly two parrallel streams.
The 20min before start you stream some nice test picture+sound maybe with a countdown. That also allows people to tune in early to avoid spikes.