If you mix it yourself, you know exactly what is in it.

This means you can vary/substitute ingredients such as heart salt for regular salt if you're on a DASH diet (half the sodium chloride is replaced with potassium chloride).

Do you know if that will even work? Baking is not cooking -- you can't just swap ingredients and get a similar result.

Yes but that's the fun part of baking: thinking you know better, substituting things, failing miserably and then either:

a- gaslighting yourself into thinking it's better than the original

b- blaming the original recipe

c- posting a one-star review on the recipe saying you replaced eggs with banana and it came out horrible

Eventually, you succeed, and get to claim you were right all along (mostly because the trash can cannot speak of your failures).

You can't swap arbitrarily but it's also not like the baking ingredients are the input to a pseudo random number generator that selects the result. You can absolutely make it more/less sweet, more/less chocolatey, more/less moist, add hints of different flavors etc.

Commercial cake mix is ultraprocessed,you likely won't find those ingredients in your pantry. The flour is bleached, they add emulsifiers you wouldn't find at home, the vegetable shortening is jydrogenated, etc.