“When there are different opinons that are all firmly justified, there is no stam halacha. That is a machlokes. Neither opinion is objectively a kula or a chumra.”
If this were true, nothing would ever be called a kulah or chumra. Give me one example of something you actually consider to be a kulah, and if you really want to show integrity, let it be one that you personally rely upon, otherwise you’re likely just making the standard MO point: “everything I do is the stam halacha, everyone to the right of me is doing chumros, everyone to the left of me is not ‘firmly justified’ (as you put it).”
Even within your current description, you depart from what you actually say in previous arguments. The Bach’s heter for chodosh is openly a limud zechus, not just an alternative opinion in a machlokes. The masses were openly eating chodosh with no existing heter, so the poskim were forced to figure something out. Same thing with cholov stam.