... the only way we know that these super massive black holes can form is by smaller black merging.
The main way supermassive black holes in the centers of galaxies grow is by accreting gas, not by mergers with other black holes. (But you can still have potential problems with not being able to accrete enough gas in a short enough time to explain early SMBHs, unless the initial "seed" BHs are larger than those formed by conventional supernovae.)
The main way supermassive black holes in the centers of galaxies grow is by accreting gas, not by mergers with other black holes. (But you can still have potential problems with not being able to accrete enough gas in a short enough time to explain early SMBHs, unless the initial "seed" BHs are larger than those formed by conventional supernovae.)