From what we know IVs don't affect catch rates at all.. i'd suggest sticking towards throwing great throws (as you have been) rather than aiming for excellent throws as their heavily bugged rn due to the curve ball 'glitch'.
At the end of the day its just RNG. Been throwing straight throws at Suicune for lols (after originally landing 34/36 excellent throws on legendaries/t-tars without captures) and have still managed to catch 3/4 Suicunes... Gamesux.
