Massive thread to local space sure, but not galaxy destroying.
What would a ricocheting bullet, at constant speed, do to the inside of a B-17, despite the smallness of the bullet itself?
Again, 5 light year black hole, larger than any black hole known in existence today, larger than the black hole at the center of our own galaxy, thousands of times larger than the largest black hole ever found, moving through the galaxy at faster than light speeds in seemingly random directions. What might it be doing to the fabric of the universe itself?
Think big. Don't think in "5 light years isn't that big" terms because a black hole the size of 5 light years is that big, it's truly pants-shittingly enormous, even in the span of our galaxy. The gravitational effects of something that large would do more than just disintegrate planets, it would change the directional pathways of entire solar systems, fling stars away from each other, scatter solar systems. A 5 light year black hole passing Alpha Centauri would obliterate us.
It would be more than just a threat to local space.
Here is a comparison chart of TON 618, the largest ultramassive black hole we currently know about in the known universe. It is 2,606 AU in diameter, which is 0.04 light years. You would need
125 of these put together to equal the anomaly in the episode.
(click it for full size)
The dot in the center represents our solar system, 80 AU across.
If this bastard were roaming the galaxy moving at faster than light speeds, it would be a galaxy wide problem. Honestly, I think the DSC folks went way too large. WAY too large for this thing. A 5 light year wide black hole that moves at faster than light speeds would do a hell of a lot of damage to our galaxy, which is only 100,000 light years across.