I don’t think the OAuth specification has any sort of capability to just ‘add scopes to an existing token’. So what you’re suggesting would be going off-specification and potentially breaking every OAuth library as they’re all pretty much standardised to what the specification allows for.
As for your trouble with UX, UserVoice would be a great place to suggest what you think should be changed and how, and any potentially additions that may help you. I couldn’t find any other feature requests on UserVoice for this so it may be that Twitch is unaware of potential issues on that auth page as no one else is suggesting improvements to be made or usecases where the current design is failing them and needs change, so if you post on UserVoice that will at least start some awareness of this.