Thanks for the recommendation, I just tried it and it seems perfect!
Contrary to your experience, I was even able to create events from thunderbird that would show up on O365. They only show up on sync however (which does not seems to be automatic, only on timer). Maybe that's the reason why it did not appear to work for you?
Re:New in Thunderbird 78
by abdullahkhalids on Thursday September 17, 2020 @01:00PM
(#24507074)
There might have been updates. I haven't tried in a while - I have to use the Meeting Scheduler in O365 to set up any meetings that I do.
RSVP doesn't also work right now from the TB calendar. Author claims he might fix it by the end of the year.