I am specifically looking for a casino that treats Plinko as a main game rather than just a side game. I want a place where there are contests or leaderboards that might involve Plinko players.

You should look for sites that put it front and center. Windice is essentially a plinko crypto casino https://windice.io/plinko considering it is one of the four main buttons in their header menu. They also have a 'Win coins in one click' feature that suggests they focus on quick engagement games like that.

That sounds promising. I like when a site prioritizes the games I actually play instead of hiding them in a sub-menu somewhere.