I can't offer any advice based on personal experience of choral conferencing, so the following is theoretical.

Zoom (based in San Jose, California) is a video and audio conferencing service I know because it's used by Mozilla for technical meetings. It came to mind because it was mentioned in the article I read this morning, because they've increased the length of free calls due to the CV pandemic.

Googling, I find that it claims superior audio quality (I expect they all do) and that latency can be a problem (it's an international service) but affected by whether you have a local 'Point of Presence'. If your singers are all in the same area, and there is a local PoP then latency ought to be low. However, as I posted before, if people are accesing it over slow mobile connections then that's going to increase latency whatever software you use. And I think you really need headsets.

So it might be worth trying. Maybe somebody local works for Zoom or you can find a forum to discuss it. I bet it's not a usage they've thought of, but if you could interest them...