Hi Anton,
I'm afraid it is not available for now to hide the system dashboard dropdown list in model driven apps.
If we add dashboard URL through page in the sit map, we will see errors when we click the page in model driven apps. Please refer to this link: Dashboard URL - Microsoft Dynamics CRM Forum Community Forum
If we add dashboard through page in the sit map, we will still see all system dashboards that we add trough page in the dropdown list.
You can create a new idea about hide the system dashboard dropdown list in model driven apps, through the below URL, then ask your colleagues to raise up the votes.
Ideas (dynamics.com)
Our product team has direct visibility of these ideas and they will implement it on the basis of the highest number of votes received.
Best Regards,
Sayen Zhang
