This app is super impressive and it seems it doesn't even have a competitor (for Google's non G Suite accounts). Did you ever think to also support syncing the e-mail messages themselves?
Currently the only way to do it is using IMAP/POP. But since Outlook doesn't support OAuth it means enabling Google's option of Less Secure Apps and giving Outlook your password, which is not a good idea. There's also the alternative of giving Outlook a specific app password, but it requires enabling 2-Step Verification which isn't always welcome.
What do you think?