Fusion doesn’t create density, though.

Density (and heat) creates fusion.

Moreover, super-massive black holes (SMBHs) are very different beasts from stellar-mass black holes. They’re not created from stellar core collapse, and, indeed, can’t be made from stellar core collapse. Stats haveaa maximum mass, due to the rate of fusion getting higher at higher masses. At some point, the rate of energy production just gets so high that it overwhelms gravity, and it actively pushes its outer layers of material away.

SMBHs are millions of solar masses, and have event horizons with radii measured in astronomical units. These are mind ending numbers, but in ways that are completely counterintuitive.

So, from the outside of a black hole’s event horizon, the distribution of matter within it is inconsequential. It’s fundamentally unknowable, and we can’t - and don’t need to - make any assumptions about where the matter is. It’s just inside the event horizon.

Now, a SMBH with 5 million times the mass of the sun will have an event horizon with a radius of about 0.1 AU, or 15 million km. This is 1.98 x 10^20 kg in a spherical volume of radius 15 million km.

The average density of this configuration is 0.00125 g/cm^3.

Any similar volume of space with a density as large or larger than that will collapse into a SMBH. In fact, it already has. The event horizon already exists.