I've always found these concepts hard to get my head around. I don't think modern physics conceives of spacetime as having a boundary in the everyday sense. But I've also heard that distant galaxies appear to be receding from us due to the expansion of space at a rate faster than the speed of light! This would make it hard to reach the boundary even if there was one.
Some things, like the tesseract are hard to describe in natural language. But imagination doesn't always conform to the limits of language - Wittgenstein claimed that "What can be shown cannot be said". Maybe this is that sort of thing.
Great post - keep thinking - life is full of mysteries.