I can host video via Google Meet and invited people can join, even without a Google Account. All in browser. No fees or account wrangling as this is provided by work (Google).
If we want to consider that, I’ll want to setup some test runs for people in representative configurations to make sure it’ll work well for people. My experience is that any existing participant can click the “allow user X to join” button within a session when someone invited but not using an account connects and that I as the creator don’t even need to remain in the session.
I don’t know if the same is true for shared whiteboard like Google Jamboard, I’ll find out. (excellent for use from touch screens)
I believe Zoom behaves similarly for non-event-hosts (I created an account when I used it but it was a free one not tied to anything - I’m not sure if the account was actually required).
For break out rooms, I suggest rather than expecting that as a platform feature we just setup several parallel video sessions that we’re all invited to. A primary one and a few others that people can self organize into for topic specific things as desired. We’re all welcome to move between them as desired (a feature of actual sprints with people in corners or side-rooms with whiteboards). But would presumably create a schedule for common sync ups in the main room and possibly some lightening talks.
We shouldn’t need to create a subscription requiring us all to have new user accounts with a per-user fee in some paid managed domain product (as MSTeams or a custom GSuite domain would presumably be) for this kind of thing. Zoom has been a poster child of enabling this so far (thus the lang. summit use). The others, if setup that way, seem geared towards less ad-hoc Managed Scenarios :trademark:. I’m not a fan of getting into that level of random company’s product admin duty myself. But we do have the PSF here…
Requirement: We can’t use anything that requires installing local third party software on a computer. It must work in Chromium and Firefox based web browsers. It must work on ChromeOS. I don’t expect this to be a problem because just about everything seems capable of this today. I expect everything has a mobile app and some may have desktop software - but use of that needs to be optional.
Meta: If you’ve got a local Zoom software installed I suggest reconsidering that install. Look into the https://github.com/arkadiyt/zoom-redirector browser extensions to protect your computer from their repeatedly demonstrated lack of software infosec quality. You can use their service without sacrificing your computer & data! 
While Zoom has contracted an all-star team of software infosec folks in the last few weeks, it’ll take time to see if they’re actually able to shape up and regain lost trust. I don’t expect to ever be allowed to install Zoom’s client software on my employers computer, nor would I expect several others of you to be able to either.
Meta meta: Vic and Vat and Wb solved these problems via the mbone for free over IP-multicast in the 90s. Caveat: Nobody had IPv4 multicast routing working and NAT killed the concept.
Disclaimer: I own shares in Zoom, Google, and indirectly Microsoft.
. Though not impossible to emulate in other tools (Teams for sure, I assume others can too).