Flaxseed oil is garbage, just use animal fat like your (great great) grandparents did. Cast iron was perfected when people cooked with lard and tallow, not vegetable oils, which are a 20th century invention.

Vegetable oil works ok too, but honestly use that trimmed fat from a beef roast or something, render it to liquid, and then season it the same way you did with vegetable oil and see how it compares.