I tried but wasn't able to find a recent thread on a different Houzz forum where someone paid $500 to have brass Baldwin hardware restored on a single doorway. I tell you this to let you decide how much diy effort it's worth to you. I don't recommend attempts at darker color and you certainly don't sand brass for any reason.

This will be trail and error until a satisfactory finish is found. The shiny spots are were factory applied clear coat remains. Dull spots are bare brass where clear peeled away. The first step is cleaning off body oils. Pros use a product called Pre-Kleeno. Denatured alcohol and lacquer thinner will work but might also remove remaining clear. Whether remaining clear comes off or not I suggest spraying new clear coat on 1 or 2 pieces without additional prep. The number of different clears available are many but for a one-time diy project I suggest Rust oleum acrylic spray can. Lacquer will protect and shine but depending on what factory finish is, lacquer might cause that remaing to wrinkle. Lacquer and acrylic both come in various gloss so read the label before purchase to make sure you are getting high gloss. If old clear remains when sprayed, examine trial pieces to see if old finish telegraphs. If factory is lacquer, new lacquer sprayed over it will blend beautifully. If lacuer wrinkles old finish don't panic, lacquer thinner on a rag will totally remove it so that you can start over.

Another option is buffing with Brasso or similar compound.

If you have question , put @klem in your reply and I will be notified. Good luck, let me know how what you think of results.