A boundary is just showing people where the door is so they stop walking through your walls.

Boundaries get imagined as walls built against people, which is why setting one feels hostile. They work more like a floor plan. Saying where the entrance is lets people come in properly rather than arriving through whatever gap they can find, and it is usually a relief on both sides.

Pick the one thing that keeps happening that you have never actually mentioned. Name it plainly and say what works instead, in a sentence, with no case attached. Most people adjust, and the ones who argue have just told you something worth knowing.