Jitsi Meet.

“Jitsi Meet is a fully encrypted, 100% open source video conferencing solution that you can use all day, every day, for free — with no account needed.”

Open Source, too. At first it appears to be Android and iOS only, but scroll down and there’s a Windows desktop version on the list. I’m not sure if it scales, so that you can reliably use it for 30 or more people for a peak-time webinar. But it might be worth trying.