For the sake of the discussion, I mean a mixture containing leavened food that people do have the practice of selling. Let’s say a very expensive whiskey. Someone who chooses to destroy it rather than sell it to a non-Jew is seemingly destroying something of value here (that money could have gone to charity, for example).

I think one has to distinguish between two different principles. In my opinion there is no issue of wasteful destruction here, because it is being done for the sake of the commandment. The question is whether there is a point in selling it to a non-Jew in order to give the money to a poor person. It is certainly possible that there is. The best option, in my opinion, is to really sell it to a non-Jew (not on the understanding that he’ll return it after Passover), and give the money to a poor person. That is completely above board, and you also have an enhanced fulfillment of the commandment of charity. But even so, if you destroy it, in my opinion there is no issue of wasteful destruction here. If there were an issue of wasteful destruction here, then even without giving to a poor person there would be reason to sell it so that I myself would not suffer an unnecessary financial loss.

I can fulfill the commandment of “you shall remove it” even by selling to a non-Jew, and not דווקא by actually destroying the leavened food. Meaning, if there are two ways to fulfill the commandment of “you shall remove it,” one involving financial loss and the other not involving financial loss, shouldn’t one choose the option that does not involve financial loss because of wasteful destruction?

Selling leavened food to a non-Jew is not removal. It creates a situation in which there is nothing to remove. True, there are opinions that this is what removal means, but in the plain sense it is a practical commandment. Although on the Torah level it may perhaps be fulfilled by nullification.

For the sake of discussion, let’s say I leave myself one cracker in order to fulfill the commandment of removing leavened food with it, and I’m left with a dilemma whether to sell the whiskey to a non-Jew or pour it down the toilet. If I choose to pour it down the toilet, is there no issue of wasteful destruction in that?

To the questioner—what is the reasoning that whiskey is a mixture containing leavened food and not actual leavened food?

As I said, I don’t think so. If you have leavened food left, you must destroy it, and there is no issue of wasteful destruction in that. There is no obligation to arrange things so that no leavened food remains in order to avoid wasteful destruction. There is some value in that, but in my opinion it is not correct to apply the prohibition of wasteful destruction here.
Similarly, suppose I was thirsty and instead of water I drank this whiskey, which gave me no added value of any kind. Even then there would still be no issue of wasteful destruction, because at the end of the day I used it for my needs. The commandment to destroy leavened food is also a legitimate need.

To By Chance:
https://www.toraland.org.il/%D7%A7%D7%98%D7%9C%D7%95%D7%92-%D7%A1%D7%A4%D7%A8%D7%99-%D7%94%D7%A8%D7%91/%D7%91%D7%90%D7%94%D7%9C%D7%94-%D7%A9%D7%9C-%D7%AA%D7%95%D7%A8%D7%94-%D7%94/%D7%90%D7%95%D7%A8%D7%97-%D7%97%D7%99%D7%99%D7%9D/%D7%A1%D7%99%D7%9E%D7%9F-%D7%9E-%D7%95%D7%99%D7%A1%D7%A7%D7%99-%D7%A9%D7%A2%D7%91%D7%A8-%D7%A2%D7%9C%D7%99%D7%95-%D7%94%D7%A4%D7%A1%D7%97/