I'll keep this short, but uploaders shouldn't be able to bypass basic meta information like that if the source 98% of the time clearly has it listed by the post title or even on-site tags.
Instead of having to tag them manually, there should be typeable dropdown menus in the upload page to select "Year" [going from the earliest tagged year + year gaps that might not be listed - to present] and "Medium" [basically all meta medium tags currently on the site]
Clear exceptions for the year tag would be for posts that have no known source, or the original work being gone but the year of the earliest repost the uploader can find.
Updated