Measuring distance by parallax would still be valid in a flat earth cosmology.
It's not valid at all. Because a parallax requires 1 distance (to either a star or the sun) to be known, in order measure the 2nd object. That's a false assumption. I think ALL measurements of planets, stars, etc are debatable because those that measure use foundational errors of heliocentrism.
That "pagan Greeks" figured something out doesn't mean it's wrong.
True, but their findings have to be taken with a grain of salt, because 1) they don't care about religion/genesis, 2) most of them are heliocentrists. Same approach to reading a protestant commentary on Scripture...you have to assume they're wrong because their foundation is wrong.