You only dedicate one tiny paragraph to the best solution: Evolution.
Evolution supports multiple accounts, which you activate via ‘Online Accounts’ in GNOME, has an integrated calendar, supports Exchange Web Server.
When you go to your System Setting, you can change both the default email client and calendar to Evolution.
I do understand why Ubuntu has GNOME Calendar and Thunderbird as the default though, as it is aimed toward your average user and because the use of an email client is declining.
For users who need this little bit more, like you, I’d say: use Evolution as your one-stop shop.