The Software Boutique is a custom boutique, strictly (kinda) for Ubuntu-Mate, that makes it basically one-click to install the most commonly used apps. It has never meant to be 'all the apps available' solution.
Thanks, I didn't know that. I used the boutique to install Spotify and the terminal & snapcraft.io for others.
As for google drive, I'm pretty sure that none of the Ubuntu flavors have something for Google Drive by default (as Google is pretty lame at making linux apps for their services)... But you may want to try OverGrive. While not free, it's relatively inexpensive at $4.99
That looks really interesting! It's cheap and it looks like it'll be worth the money.
For now I 'solved' it by installing Gnome control center and online accounts with:
sudo apt install gnome-control-center gnome-online-accounts
After that I could start the control center and configure the Google accounts with:
XDG_CURRENT_DESKTOP=GNOME gnome-control-center
Edit: I just noticed that adding my Google accounts with this method also added the account in Evolution, so I have access to my mail, calendar, contacts and tasks.
This is a lot more user friendly than adding the services one by one in Evolution.