How can i define 'default permissions' for new files in the 'Billing export bucket' so that they are also in read-only for Teevity ?

Click on "Storage browser" service, down at the bottom of the left navbar.

Click on the 'triple dots button' at the end of the line for the bucket you want to configure , then click on Edit object default permissions in the menu.

Add the 'Teevity ServiceAccount email' as a User with Reader permissions
Teevity ServiceAccount email address 985668681564-9257rkc0bnb09l6e5228bb1ornpt0942@developer.gserviceaccount.com