Faded colors of old slides - how to improve?

There is some information in this thread on how to restore slides.

For what it’ s worth, I believe the original picture corresponding to the slide you show was actually a little overexposed to begin with (the sky has zero detail- could also be heavy overcast weather), which means you can only get so much back in restoration from the slide. I’ve struggled with this too, see e.g. here.
Your second attempt is already quite nice!