Hi there,
we also have the worst summer ever, almost 2 months with AC on 24\7
When my LO was with us in our room we always had AC on and she was in a sleeveless onesie and a thin blanket - otherwise she would sweat during the night but that's something we learned over time.
When she moved to her own room she has a ceiling fan on all night, and we only cover her with a very very thin cloth (like a big Burp cloth).
I turn on the fan and put her in a sleeveless onsie and a very thin sleep sack (otherwise she gets her legs trapped between bed posts).
I would think a sleep sack just makes it hotter wouldn't it? for us we use a bed bumper all around the bed so her legs don't get trapped.