(Trick question) explaination below
contrary to what the fanbase believes Isshiki was already at FULL POWER. That’s exactly what the problem was! When Isshiki reawakened in jigen he was brimming with SO MUCH POWER his body couldn’t contain it and would crumble from that power in 2 or 3 days. To put it simply He didn’t need kawaki to make him full power, he needed kawaki to contain the full power.