You can use Jitsi for free at https://meet.jit.si. It’s a free service. For more control over your meeting environment, you can subscribe to 8x8. They’re both the same service, but an 8x8 subscription grants you more control over the environment.
There is no scheduling per se in Jitsi (you can add a reservation system if you deploy your own server). Meeting names become part of the meeting link. If you choose a really unique meeting name (that’s hard or impossible to guess), then you can share that link ahead of time with your intended attendees. Meetings are actually created when the first person enters the meeting room (so in effect, the room does not exist if you send a link before scheduled date/time).
It’s possible to send guests a meeting link they can add to their google calendar. There is no RSVP feature.
There is no limit on the duration, however, one conference can host a maximum of 500 participants.
Well, they would first need to connect their google calendars. So, for instance, if you go to https://meet.jit.si, you’d see an option to “Connect your calendar”. Once they do that, they can add whatever link you send them to their calendars and can just join the meeting with one click from the calendar.