Hello Dewey Wu
Good day and thanks for posting in Microsoft forum community, it's our pleasure to assist on the above query.
Regarding calendar reminder plugins, by searching, there isn’t any official plugins currently, we are sorry for the inconvenience caused. However, you can adjust notifications via clicking your account tile on the right-upper corner->Settings->Notifications, for meeting started notifications, there are only two options: Banner or Off. Regarding your concerns, I understand that this may not meet your needs, you are welcomed to share this with our related product team via Teams UserVoice: https://microsoftteams.uservoice.com/forums/555103-public we appreciated every customer share their suggestions for the products. These suggestions are considered usually for future productions.
Thanks so much for your time and have a great day and stay safe.