I eat fast food multiple times a week

This doesn't mean anything. Just like improving your climbing, what matters is consistency. Count your calories and macros and you'll see that your total intake is reasonable. The processed vs unprocessed argument is negligible when you're only partaking occasionally.

When I was younger, I couldn't understand why I was thinner than my friends. We'd go out to eat and I'd stuff down a huge meal with the best of them. Turns out that they were eating like that at every meal, while I was having cereal for breakfast and a less calorie-dense lunch or dinner.

as you build muscle, your rest calorie consumption increases

The difference is tiny. Yes, you can outrun or out-muscle a bad diet at the extremes, but that's like saying BMI isn't a useful metric. You're not 1970s Schwarzenegger or Phelps.

> BMI isn't a useful metric

People these days are generally also significantly taller than malnourished Belgians were back in the 1840s. That skews it a bit since the formula itself is still the same.

I guess it's still a marginally useful metric in some case but now that when can accurately measure body fat, muscle weight etc. there is no point paying attention to it that much.