"Polyester will melt from a hot iron"
I didn't find that it did when I made many string blocks using up old half empty spools of poly thread. I did press the strips with a hot iron as I added them onto the backing, and also pressed the backing where the thread showed and never had anything melt. I've also noted that many long arm quilters use poly thread.
I usually use Connecting Threads thread in neutral colors for piecing, love the light gray as it goes with everything.