Peanut butter and marshmallow fluff tend to be exactly the kinds of things that are stocked on the shelves in the "American" section of grocery stores in Europe, along with some other oddities like crispy taco shells and a few types of American cereal. So, they might not be as hard to find as you might think anymore. But, they're certainly more expensive there! I think Kraft Mac n' Cheese could be a good option, that's ubiquitous in the US but I've never seen it in Europe.
That being said, when I went to eastern Europe pre-pandemic, people requested Reese's Pieces specifically. Not reeses cups, just the pieces.
Old bay is a good idea, along with any Tex-Mex ingredients (taco seasoning, etc).
Note that packages are taking an absolute eon to clear customs right now, so be sure to pack something that is very shelf stable.
Found this:
https://www.expatfocus.com/columnists/courtney-martin/the-best-american-foods-you-wont-find-in-germany-1494/
and this
https://www.reddit.com/r/germany/comments/5fpvb2/snacks_that_are_not_available_in_germany/
Both of which mirror a bunch of the comments on this thread. BBQ sauce is a good one.