You bring a lot of concerns with very little facts or data to back it up. You should do the research on all of these things because every single one is a case of nuances.
First, fish consumption mercury concerns are mainly relevant with large fish. So stick to smaller fish like salmon, or, if you want the biggest bang for your mercury buck, the healthiest fish to eat is actually anchovies. And frankly if you learn to prepare them right, they are delicious.
Regarding juice, you don't need to drink much juice to get a vitamin D boost. First of all, at least around here, the only juice you typically see with it is orange juice. A glass of OJ a day with an otherwise recommended diet isn't going to give you any problems unless you have an allergy or something. Actually a glass of OJ without supplements is not a bad idea as many popular "healthy" diets lack citrus which is important for numerous reasons. But the key here is quantity. It's true, lots of juice is associated with bad health outcomes. Just stick to a max of 1 cup (~230ml) a day, and be sure to get sufficient fiber. I'd always eat something with 5+ grams of fiber along with my OJ. That said, if you prefer you could always just drink milk or take a supplement. Or go outside. I get my Vit D by going outside. I drink OJ daily because there's very little citrus in my diet otherwise. Plus I like it. It balances my black coffee nicely.
Cereal, again, depends entirely on what you're eating as well as culturally what you even consider cereal to be. There is a near infinite range of foods called cereal with highly varying degrees of nutrition. I personally wouldn't rely on cereal for nutrition. The nutrient rich cereals on the shelf at the store are basically typical low nutrition grains with a ton of added vitamins. You may as well take a multivitamin. The importance of mentioning cereal is that they already have many added nutrients so you should be paying attention to what you are ALREADY getting before choosing to add a supplement.
Honestly, all of these things are VALID concerns, but, your whole post is an exercise in modern blanket nutritional and health guidance. In short, blanket health guidance is crap. For any blanket guidance you encounter, even scientifically support guidance, you are going to have a plethora of nuances that undermine it. Just take the "avoid carbs" advice so popular today. Turns out the healthiest people get about 50% of their daily caloric intake from carbs. But it's the KIND of carbs, and the combination of other factors that matter the most. https://www.medpagetoday.com/primarycare/dietnutrition/74608 In another recent study it was found the Japan's oldest and most healthy elderly people lived on 80% calories from sweet potatoes, ate hardly any rice, and small amounts of meat/fish. Sweet potatoes are extremely nutritious so I'm not surprised. The carbs are mostly complex as well. That's pretty important.
Point is, be sure to understand the details or you'll end up blanket statement-ing yourself out of every good thing this life has to offer.