The theoretical max payload (that is, what the engines and their fuel will throw into orbit) is or at least has been constrained by the structural strength of the core boosters.
Crossfeeding was also considered for the Falcon Heavy, but was rejected in favor of underthrottling the core until the side boosters were shut down (or expended) and jettisoned.
Some cool innovation! Love it.
Now here’s a double irony:
1. SpaceX launched 24 satellites for Amazon’s Project Kuiper broadband constellation — a competitor to StarLink!
2. Bezos has to depend on SpaceX to launch his satellites and cannot use his own Blue Origin rockets.