Please implement a feature to send participants a choice of different appointment availability time frames. So far external tools like Doodle are needed. Why not implement directly a smart solution into Outlook?

Offer the requesting user, to choose different dates and times, to add them up into one request to participants. (similiar to your series function already, just to choose a bunch of flexibel dates instead of just serial behavior)

the participants then should get all requested dates into their calendars (shown like non answered- dates yet) - when opening the request (no matter which of the single ones), offer a display of all requested times and let the participants choose as in a multiple choice.

the user who sent the request should then get a display with the combined result of all returned availabilities. etc