I literally chill til the very end of boarding before I get on. Like you said, my seat is assigned, it ain’t changing, and I’m packed light enough to put my backpack under the seat in front of me. Sometimes people don’t show up n I sneak into a better seat 🤫. I’ve only ever been called out for that on Frontier, they know the hustle apparently. I’ve never bothered trying 1st class or anything, but I’ve gotten some middle seats to transform into aisles that way.