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"
If you are not a premium user of Todoist, how can you do a backup? Create backup isn't an option in settings
Unfortunately, this feature is not available on non-Premium accounts, though please note that on our end we fully back-up all data multiple times a day for all accounts.