Forced to make all users Team Admins? "You must be a team administrator to authorize this app"

This error is about which scopes are being requested. For reference, scoped apps can use two main kinds of scopes: "Individual" and "Team".

Individual scopes enable access to functionality for an individual account, such as uploading and downloading files, creating folders, etc. They can be authorized by any kind of Dropbox account.

Team scopes enable access to functionality for entire Business teams, such as managing members, etc. They can only be authorized by team admins.

The error here is indicating that the app is requesting team scopes, but the user is not an admin and so cannot authorize that. If you want to allow non-admins to connect the app, you'll need to request only individual scopes. 

If your app never needs to use team scopes, you can configure this by disabling the team scopes entirely from the app's Permissions settings (which are shown in your screenshot).

Alternatively, if your app does sometimes need to use team scopes, you can instead specify which scopes to request from a user on the fly by setting a particular set of scopes when constructing the authorization URL. That is, you can list the scopes to request in the 'scope' parameter on the /oauth2/authorize URL. (If you're using an SDK to process the flow, refer to the relevant SDK documentation for information on how to configure that.)