Hi @lharnisch,
The easiest trick might be to redirect members to the profile edit form after they log into your website. You can use Login With AJAX to apply these settings to your website. For more, please refer to the following topic:
https://support.themosaurus.com/forums/topic/auto-redirect-user-after-registration-login/
Another way to solve this might be by adding some kind of alert message (https://getbootstrap.com/docs/4.0/components/alerts/). As our theme supports Bootstrap 4, you could use the same HTML tags as the demo and replace the text by your warning message. However, to write the PHP conditions necessary to display your message, you will require custom development work.
We recommend you to require the services from Envato Studio freelancers (https://studio.envato.com/explore/websites-programming). This web platform gives you access to developers who have already experience with theme customizations, and for very competitive prices.
Best regards,