You use an insecure but accessible option? You have a chance at organizing successfully but you risk union busting.

You use a secure but inaccessible option that people don't already use? You have no chance at organizing.

Discord is where the people are. As an organizer, you don't get to decide for people where they should be, you hold your nose and use a secure option for the organizing committee.

What would you suggest instead and why would it have changed the outcome?

A whole lot of people working in tech are not themselves using the bleeding edge of tech