Culling should be pretty straightforward. I suggest using flag or reject for images that you don’t want any more. Both can be done with gestures on the image (swipe up or down). That information will sync over iCloud Photos. If you are using Files, then you can’t sync the information.
There is no feature in the app to convert an image into a DNG. If you just want to export the original, then use the Export feature and pick Original from the first popup. You can choose Photos as the destination.