Why not silence for seven days? That would have been cool.
The half hour has to mean something. Right? It just seems so odd to have a period of time so mundane and realistic that it doesn’t fit with the book. “Yeah, you should have been there; we had living beasts with animal heads, cherubim, Jesus (first fiery, then lioney, then a lamb), and then my shoelaces came undone so I had to tie them again.”
My intention is not to mock, but to illustrate my point: a half hour just doesn’t seem to fit here. The sight of Jesus, earlier, caused a fall-down-dead effect. That tells us how awesome He is. Here though, the seventh and final seal gets treatment that, apparently, Schubert would warrant.