I am slightly concerned that there is no way to back-up my data to a 3rd party data storage service.
As a premium user, your data is backed-up on a daily basis. You can also do a manual backup. Here's how you can do it:
1.) click the gear icon in the upper right corner
2.) select "Todoist Settings"
3.) go to the "Backups" tab
4.) at the bottom, click on "Create a new backup"