You're right you can't all hear each other. Some choirs use instructions like https://www.makingmusic.org.uk/resource/zoom-online-rehearsa... where everyone is on mute and can just hear themselves and the choirmaster.

I think a technological solution would exist: A and B hears choirmaster, A and B's sound is sent back to server which syncs it with the choirmasters sound before sending to C, D, E and F. C, D, E and F's sound is sent back to the server which syncs it and sends to G, H, I... etc.

So most people can sing along hearing some of the choir and the latency overall shouldn't be too bad. Scales with log(n) * max(latency)