How to decrease stretch even more?

Reduce Format > Page settings > Scaling
Or use wider paper

If you can't decrease stretch further, then it is because you have reached your minimal tight spacing setting as per a whole bunch of other style settings.

In your example it's not the notes but the melisma lines that for some reason reserve so much horizontal space... If you would delete all lyrics, all bars fit into one system ;)
You could distribute the lyrics onto more bars...

As mentioned, stretch works only to the limits of your other style settings - the ones in Format / Style / Measure. So, you can reduce minimum note distance and others. But in general, this usually produces an unpleasantly crowded look. Simply reducing the staff size is usually the better method. Vocal music typically uses slightly smaller staves than instrumental (because singers will typically be holding the music themselves rather than using a music stand), so you can afford to go down quite a bit.

In your case, though, it's the spoken lyrics that are the main issue. Since there are not "true" lyrics (not because they are spoken, but because they are entire phrases on a single note), probably better to just add them as staff text, so they don't mess with the spacing algorithm. Stretch settings are about note spacing, but lyrics take the space they need regardless.