Dust is an issue. One thing I have seen with many old slides--especially those processed by Eastman Kodak an, yes, Technicolor, are scratches. Interestingly, what is often scratched is the lacquer EK/TC used as a protectant. You can remove the coating and remove most dust as well as most scratches.
There are several approaches: use a camera and macro lens; use a dedicated high resolution scanner, or some other device.
Or, you can take a photographic approach and do things old school. Sadly, the proper materials are no longer made and the process becomes daunting when your collection is vast.