Yes, what you say does happen, whales sell 20,000 BTC, people panic, price tanks and then rebuy cheaper. However this is very risky because investors are getting more sophisticated and when it dips they buy instead of selling. I’m sure sometimes those whales have caught their fingers and rebought higher instead of lower.
But even when they succeed, it’s just a temporary dip that you could use to your advantage. Besides, the really big whales like Microstrategy and Grayscale never sell.
About the futures, they often try to short the market but usually they get it wrong and get liquidated. Recently they were betting for the price to go to $20,000 and it went up, they lost a lot of money there.
So, I would say, ignore the shorting and when there is a dip, buy.