Spam musubi is identical to your pumpkin pie example, main ingredient changed and often with different sauces/sides/etc like spicy mayo, teriyaki or gravy. Either they’re both unique or neither one is and, based on how you categorized everything else, nothing is unique.

Loco Moco is just egg curry with hamburger.

Gumbo is just bouillabaisse over rice.

Bolognese sauce on the other hand is, as I can’t find any other similar dish that was invented independently

Tomato based meat sauce? That’s just curry with some of the complexity removed.

Poutine nachos? Authentic Mexican food!

Also, think about what it means when you dismiss a food as “uniquely American” because it’s “Native American” cuisine.

Midwest and Alaskan, as well as east-coast, those three sound most promising. Can you maybe tell a bit more about them?

I’m sorry to say, but there’s nothing unique in any of those places either. Ambrosia is a standard fruit salad, Cincinnati chili is just spaghetti and hot dish is just shepherds pie. Sloppy Joes are just a ragú curry sandwich and corn dogs are tamales on a stick. Akutaq is just ice cream with an extra ingredient or two and birch syrup is “an ingredient, not a dish”.