Question: what ilvl should i be for mythic dungeons?
Answer: The game does not prescribe a minimum level. Mythics (M0) drop 400, so we might reasonably expect that 385 would be a minimum practical ilevel, but gear is very easy to get up to about 410 from world quests anyway, so once you've opened them and picked up some 385s from Nazjatar, you're well on the way.