The main problems I hit with repairing lenses are:

The ball bearing which provides the aperture click - you need to watch this like a hawk or it will fall out, either bouncing onto the floor or sticking to something unlikely depending on whether it has any grease on it.

Cemented element groups where the fungus has got into the cement. Short of having the parts separated and re-glued (a really high-precision job for a professional) there's nothing that you can do.