Better 7-speed chain for Trek Allant?
First: based on your measurements, your chain is not worn out. The park tool gauge measures a 8 1/2" links, or 4 inches of chain. It looks for wear between .5 and 1mm of "stretch" in the chain. 

If you read the label on the tool, anything up to .5 is what a new chain wil read. A chain is not worn out until it hits 1mm of wear, which is when it usually will skip teeth on a new cassette or chain rings. 

To protect you old chain rings and cassette, it is usually recommended that you change the chain at around .75mm of wear. 

Second: Shimano's recommendation is that a new chain up to the .75mm mark is good for 880 miles on average. I assume that recommendation is conservative, because it often goes to double that in my environment, but speed of wear is based on maintenance and environmental factors like heavy wet weather, and can vary wildly. 

I hope that is helpful.