I still don’t get it. It doesn’t really make sense to me. If it takes a lot of focus and concentration to maintain the solid form, why is one considered weak for doing so?

They seem to be saying that the solid form is a sort of defense mechanism, like a snail shell or an opossum playing dead (or maybe an environmental one, like that it prevents the jelly form from losing too much moisture in a warm environment). It's difficult to maintain, and implies you're in a position of retreat or weakness. Now that the Breen presumably have no predators and no environmental necessity for the solid form, it's seen as a cultural taboo.

While I'm a little bummed the Breen aren't the space-arctic-wolves I imagined them as during DS9, I think it's an interesting idea. I do always like when they describe how cultural practices in a particular species comes from how they exist in the ecosystem of their home planet, like the Kelpiens (Saru and the Kelpiens being for me, Disco's most successful addition to Trek canon).