No, I don't substitute butter. If I want to use butter I buy a mix based on adding butter. I generally prefer the texture of the oil-based cakes. (Personal preference, obviously.)
When I want a somewhat more dense, sturdy cake I use the very-old very-standard "pound cake variation" which is to add 1 4-serving size pudding mix (dry), add one extra egg, and increase the oil a little if the mix calls for less than 1/2 cup of oil.
That is true of Pillsbury and Betty Crocker, too, mmumsie. A half-century or so ago, when my friends and I "doctored" a cake mix it was to fix some flaw -- to make them well.
Cake mixes all on their own make perfectly "well" cakes these days. No need to overcome a cardboardy taste or mask the coconut oil flavor, etc. And certainly not to make them moist -- they can teach scratch cakes a trick or two in that department.
It is still fun to "doctor" mixes, for changes in flavor or texture and just to feel creative. If you want an orange cake, sure, change the liquid, add zest, maybe experiment with Grand Marnier. But for a plain yellow cake? The cake mix is fine just as it is, thanks.