I would get a hammer and trigger set and clean up the insides well. It sounds like rebound slide is gummy or spring is weak. On the timing. Does the dun have much end shake? Press cylinder to rear and slide largest feeler gauge possible between cylinder and end of barrel, then repeat holding cylinder as far forward as it will go. Subtract measurements. If more than .002 its to much. End shake can cause poor timing. Get some shims and install looking for .001 end shake. If that don't fix timing you should get a new hand fitted. If only 1 or 3 chambers are slow you could peen their teeth.

I would take off the grips and soak the whole gun in a tub filled with transmission fluid and kerosene 50/50 for a couple days. That will loosen any rust or crud and may well free up your rear sight.

Finish. Remove grips, rear sight assy, hammer and trigger. Install side plate. Plug both ends of barrel and the opening for hammer and trigger with silicon caulk. 1/4" deep or so let harden. Take it to a sandblast booth with glass beads and Have at it until it is pretty uniform. Parkerize or rust blue. Rust blue. Find a big box or make one of wood. Make a stand using a dowel that goes in the muzzle and holds the gun up. Stick in a light with a regular light bulb about 60 watts and a wash cloth that is damp with saltwater. Leave the gun in there a day or 2 until is light brown with a light coat of fine rust, wipe it off with real fine 0000 steel wool. Stick it back in box again. Keep doing that and wiping it off with the fine steel wool. After about 10 times or until the brown is real uniform after the steel wool wipe down. Then, stick it in a pot of boiling water for a 1/2 hour or so, this will darken the brown to blue/black and fix it, then oil it real good. It won't fix the pits, but the glass bead blast will help the surface look more uniform. I bet you will be surprised by how good it comes out.