RG:
You wrote: “the Torah biblically MANDATES and OBLIGATES on all of us to take on chumras, to REFRAIN from things that are muttar!!”
You are 100% correct. Where you go wrong is that your choice to refrain from something enters it into a category of issur, that someone that makes another choice, and doesn’t adhere to the chumra you have chosen is being “meikil”. That is blasphemous. The Raavad suggests that we leave over some foods on our plates, as a way of refraining from the muttar. It’s an idea, but it would be baal tosef to claim that not doing so is a kulah. This is an instance where we make personal choices of refraining from “MUTTAR”. If I choose to not eat cheese on Fridays (yes, there is such a minhag), that can be mine. You are no less frum if you do eat cheese on Fridays. It never entered my mind to inquire whether someone was as frum as me if they shared the same minhag as me. The idea is not about concocting more mitzvos, but in our exerting control over ourselves so that we do not get swept away by the worldly desires. What we choose is not a matter at all of bein odom lachaveiro, and has zero relationship with the kadeish atzmecho of another human being. If I do not do what you do, how does that make me wimping out on Ratzon Hashem?