I have digitised 1000’s of negatives and slide films from the 80's & 90's using the Konica Minolta DiMage Scan Dual IV. (not a flatbed).
I have been using this for the past 10 years scanning at 3,200dpi on both, PC’s and Mac.
Recently, I used a friends Canon9000 flatbed scanner to scan both negatives and paper and was shocked to see how poor the quality was! for both, negatives and paper!
I have used Canon 5D II for scanning purposes and found it to be better than the Canon Scanner for negatives, but not quite as good as my trusty Minolta of 10 years ago!
Perhaps you should try out a dedicated negative scanner first as I believe they have come down in pricing now and may have improved on performance too.