I've done my research and know the word counts currently expected for MG manuscripts.
I focused on filling plot holes and refining characters before looking at my word count.
Now line editing comes in. Once the story was solid, I loved seeing how every word cut counts, how the story got tighter, leaner, meaner, and the tension heightened when I removed repetition (which I dearly love!) and my sentence construction became shorter as the tension heightened.
Line editing is truly where the "magic" happens.
I'm lucky I was a newspaper reporter, trained to write to the size of the "hole," and later a newspaper editor, who slayed babies ruthlessly to fit inch counts.
I'm also lucky my alpha-reader is a former journalist, skilled at cutting and tightening and strengthening with fewer words.