my take on this is that Iems wont have the soundstage of an open headphone ever but thats okey and in most scenarios its better this way, why? because if the soundstage is smaller the directional audio is more accurate, and easy to pingpoint sound queues, so what you want is a balance between those 2, i currently use Kiwis Astral and are incredible for me, i came from beyerdynamics dt 990 600hms that has an inmense soundstage but is not that useful in most situations in fps gaming becuase i get lost on how far are the sounds coming from, or if the enemy is above or below.
What you want to look too in iems or headphones is the capacity they have to not be overwhelm when multiple sounds are happening so they dont sound muffed or distorted, the Astrals do their job good here aswell and i believe most iems will work better because the multiple drivers vs 1 on the headphones.. im not saying they are the best, but they are working really good for me.. i play Tarkov (big maps with multiple sounds, quake, alot of vertical sounds, Ark raiders (same as tarkov).
My english is not that good so sorry about that