Soaking is a dial, not a rule: five to ten minutes softens the cheese without doing much to the salt, twenty to thirty (as in the recipe) gives a noticeably milder slice, and a few hours with the water changed once genuinely de-salts a very salty block. Whichever you choose, drain and pat it thoroughly dry, because wet halloumi steams instead of browning.
Eat it straight away. Halloumi is soft and squeaky hot and turns rubbery as it cools, and reheating does not bring it back. Have the flatbread warm and everything else laid out before the cheese goes near the heat.