>the entirety of the universe becomes a dense point which condenses into an infinitely massive black hole? Which then collapses and causes a big bang event.

While theoretically possible, we don't see enough observable evidence to support this.

Additionally, think of this. At what specific point of size/mass would a black hole actually "explode" into another Big Bang? As far as we know, each black hole that currently exists at any size or mass is already infinitely dense. Even if you combined all the matter in the Milky Way Galaxy into one black hole, it would still be an infinitely dense black hole, it can't get any more dense than it already is. It *would* become more massive and have a larger event horizon.

But there's nothing fundamentally different about a solar-mass size black hole and a galactic core black hole, aside from just being much much more massive. Again, as far as I'm aware.