"hydrogen was the only element in existence, and it was cracked by fusion in early stars into helium"
Large amounts of Helium were formed in the very early instants of the Big Bang, long before stars appeared.
"and then into ever more complex elements that make up life today.
It appears that the young stars in the early galaxies like EGS-zs8-1 were the main drivers for this transition called reionization "
Reionization was the early stars emitting enough radiation to ionise the neutral hydrogen gas. It's nothing to do with nucleosynthesis.