@Griffin-McPherson you're welcome!
So here's my take on it: it's not about style. Picture books are illustrated in a variety of styles. It's about subject matter. Will it draw kids in? Will it interest them? That's what makes an illustration fit the picture book audience.
Usually -- not always -- picture book illustrations are about things that young kids love, are relatable to them, and/or are part of their lives. If the illustrations feature kids, the kids are usually around ages 4-8. (Of course, there are always exceptions to the rule. These are generalities.)
I think that's why your portfolio feels like it fits more in the graphic novel / middle grade genre. The subject matter of many of your portfolio pieces feels like it's for an older audience, and many of your kids in these pieces look to be 10 years or older. It's not the style. It's what is being illustrated.
This isn't a bad thing. If you are drawn to illustrating things that fit more in the graphic novel or middle grade genre, that might be something to consider. Maybe those subjects resonate more with you? Maybe that might be a better fit? (You can always do both; they're not mutually exclusive.) If you really want to be hired to illustrate picture books, think about adding more "picture book subject" illustrations: families, home life, school scenes, animals, things that would resonate with a young human...
To prove that your style CAN be a fit for picture books in the current market, I did the exercise you're talking about. Below is a screenshot of one of your portfolio pieces surrounded by 8 illustrations from traditionally published working illustrators -- all have had books published within the last year.
When I look at this, your work fits right in. It doesn't stand out as amateurish. You've got the skills. You could do this!
This is just my personal opinion and I'm not an agent or editor or art director... I think that your current portfolio isn't showcasing your work for the picture book market as well as it could. Consider adding more illustrations in your portfolio with the fun, imagination, and whimsy of this pig with the basket of kids on its head.
Another LONG reply from me! Hope you found something helpful in here somewhere...