Multiple simultaneous meetings that transition to one webinar

Does anyone know how I can best accomplish the following:

We want our association members to register for a webinar, but before the webinar begins the members would first start in their local chapter meeting for 15 minutes. In other words, we would like to have multiple Zoom meetings running simultaneously (with pre-assigned members) where participants can see and communicate with each other (like a meeting), and then at a pre-scheduled time (15 minutes later) we want to bring everyone together from the separate meetings into one giant webinar, which is managed by a host and designated speaker. During the webinar portion, we do not want to see all the individual participants nor allow them talk (at this point only the host can do an introduction of the speaker/presenter and then the speaker giving the lecture would be seen on the screen along with their presentation/slide deck. During the webinar, participants can post questions (moderated) and participate in polls. As you can see, we are basically starting with multiple smaller meetings for 15 minutes and then bringing everyone together into one webinar. We’d like participants to register for the webinar so we can track they actually attended it. We want both the registration and meeting to webinar transition to be as seamless and dummy proof as possible.

Any help would be appreciated!