Advice on going vegan as a teenager with health issues?

I am a high school junior who is already pescatarian but would like to transition to being almost entirely plant based. I am trying to be more environmentally conscious in all areas of my life, but diet is one of the areas that has been really challenging for me. The pricing of vegan specialty/prepackaged foods is somewhat out of reach considering my family would be buying both vegan and non vegan foods,meal prep is an accessible option but might prove time consuming. I'm also worried about getting enough iodine and selenium in my diet (i need a lot of these minerals to regulate thyroid issues), which I normally get through sustainably sourced seafood. If you have relevant tips, or have experience with going vegan at a younger age or hypothyroidism please share, it would be very helpful.