[Deecart (OP)]

However, the universe is not old enough for the stellar-mass black holes to grow into supermassive black holes (millions to billions of times the mass of our Sun) by individual collisions
- So it is a mystery how the supermassive black holes grew so fast in the early universe, forming the quasars that are visible at high redshift

Yes you are right.

But perhaps this anomaly will be explained soon by some more advanced physic (some extension of the general relativity, the JANUS model of Jean-Pierre Petit).
The big bang model do not hypothesis negativ mass (some physicists tried but failed because they supposed that negative and positiv masses would have some run-away effect).

The paper can be found here : https://www.jp-petit.org/papers/cosmo/2022-HAL-incoherence-trou-noir.pdf
It is actually in french language and some people are actually working on the english translation.
It shows in the first place that the black hole object is not well understood because of initial errors into the schwarzchild formulas and because the full work of schwarzchild is unkwnown by most physicists (because there was no translation until 1990 from german to english after the dead of swharzchild of his second paper...).
In my opinion, every cosmologist should have at least read this new paper so as not to continue the errors.