If El Savador actually sell their Bitcoin, the market confidence will definitely drop and the bitcoin price though the chance might be low, it might follow. However, because with recent Bitcoin ETF bloom and Bitcoin adoption is getting wider the market will recover in short time. While the breakthrough by El Salvador making Bitcoin as legal tender is influential, but if the US is doing the same thing, that would have a much greater effect given the nation's economic power and dominance in global financial markets.
The point is, even if El Salvador sells bitcoin, the impact may be short-lived, it is natural to sell in large quantities, but after that bitcoin will recover even if Michael Saylor sells bitcoin it will be the same. Because the limited amount of bitcoin is what makes bitcoin survive and will even become a rare asset in the future.
So the impact may be short-term, but after that there will be more people who want to buy bitcoin at a cheap price for the long term. Btw what you said is right, of course the influence of El Salvador and the US is different. If the US did what El Salvador did, the impact might be a little different because the US is the center of the world economy and they have a big influence on the world.