Unfortunately, there isn't a way to make device category selection a requirement. I've complained about some of these issues to Microsoft before, but it might have fallen on empty ground.
My recommendation is to NOT use device categories for dynamic groups unless you really have to, especially if your organization uses Azure AD. It's also very fickle since some users will choose whatever not paying attention to what they select, and they cannot go back to change it once they choose continue.
You can take advantage of dynamic user groups to push applications to most devices using variable fields like "Department". The other way is to manually check using filter to narrow down "Unassigned" devices. However, you also have to manually assign each device one by one to your desired category. It's needlessly tedious. Even if you have a script for the Intune Graph API, you still filter out devices separating what goes where.... I think you get the point.
