Your arguments is based on the idea of what it's ingredients are and going, those are easy to acquire, so it must be easy to make despite almost every point made within the game saying otherwise. If it was that easy Ella an established genius at this sort of thing wouldn't have needed to break in, Deryl wouldn't have needed so much time to make an inferior version, despite having direct access to the original, Bramon himself said they make time to make and his son, another established super genius said he can't do it. Yes in real life you can replicate someone else's work easily enough with the knowledge and expertise, but Bramon is a brilliant man in a superhero setting, that often means they are essential in making them.
HERO's morales have already shown they would allow it, they are willing to empower willing people, despite their being a small chance of success and a higher chance of death or worse and that many of the Superhumans for Hero are desperate for that serum and would take the risk.
Then you have Sin who only seem to have access to the inferior monsterisation version, this version is only recently perfected, it's clearly more than just shoving monster blood in a syringe.
Ella isn't a genius, she has a lot of knowledge about monsters, their powers and apostles that she either learned herself or was taught, if the serum or its equivalent was already used somewhere, she probably would knew. But to our current knowledge it was not, Bramon was probably the first being ever to develop it and apply to others. Not that it didn't happen before, but that it was made not on accident, and recognized as such. Consider MC - his third evo was triggered by the similar mechanism as his serum, lots of different monster material, but MC didn't even realize that until Bramon explained it to him. Without that it would probably be lost to time. Now, the serum is lots of monster parts in a syringe, and as evident by MC reacting to exactly that, if you're lucky it works without monsterification, but I think the intent put into it is the exact ratios and preparation, with over a hundred different monster species mixed finding the one needed for an individual is the problem and time-consuming part, not the making it at all.
And yeah, I'd agree his expertise and, well, his very own brain and mind was essential in developing the serum over the years, that when you already know how to do it, then it can be taught. And Tanos said he can't make it, true too. But, do he know the mechanism behind it? He very well might not, and that's what is stopping him from making one, certainly not etics or lack of monsters. Deryl might know how to do it, but the way of determining the ratios or some other mechanism wasn't given to him by Bramon. Why wouldn't he do it then? Well, why won't anyone else gamble someone's life, their close friends to boot on something that might quite possibly fail. All speculations on my part, yes, but not without premises.
