My nephew went veggie at 13, and I always think of him when people start advising cookbooks and recipes and batch cooking.
My sister's family really doesn't cook - lots of oven chips and frozen burgers and food from tins or packet sauces, stuff that can be put-together or heated rather than cooked. Fruit rare, vegetables even rarer. I'm not judgy; that is just how it is for them. Everyone seems healthy enough, if a touch overweight.
So he swapped in veggie burgers, okay, but you can understand how freaked out my sis was at him going down thus very odd path in a household that was just not set up to support him. It was a real shocker and frankly he spent 3 years eating oven chips and cheese and the odd Quorn whatnot.
So, even as a lifelong veggie myself, I know how very very difficult this might be for some families.
My nephew is now vegan and a fabulous cook and never touches Quorn or meat substitutes. But it was all a bit of a tough period and there were lots of tears.
Good luck, OP - you really do need to support them in this. It is a chance to expand your horizons and learn new foods to love. What else are dc for if not to force you out of your comfort zone at every opportunity?