Is simplication of known results worth a publication? I am an applied mathematician. A very simplified outline of my situation is as follows:

The leader of my research group is a "big wheel" in his specific research area. His contributions are considered both important and very demanding.

Recently I found a way to get a simplified access to one aspect of his work. I am very sure that no more than a few dozen people have ever made the effort to dig through this and understand it. The simplification is substantial, and even to my group leader it has not been known. Certainly it is not known to almost everybody else.

Other than an alternative simplified manner of setting up the theory, I can round out some things that have previously been done by hand-waving. That is only of theoretical interest.

So I wonder: finding this has been difficult, and it would help other researchers to know about it. But is it worth being published, with only a "sugar glaze" of actually -new- results? How can I determine this? How low for a journal should I aim?
In general, this kind of result might be worth publishing, also in top journals. However, it is impossible to tell without knowing the specifics. It might well be something that has been known as a "folklore result" for long in your field.

Luckily, you say that the leader of your research group is the major expert on this. So, ask him and not a random bunch of people from other fields you found on the internet.

If you have already asked and he disagrees, chances are that he is right. But if you disagree and insist that it is a major breakthrough, you can still write everything up, put it on arXiv, and wait for feedback from other researchers.