For those who have played a game on epics store
* Can you play the game without the launcher open once you buy it ( is EGS a drm layer)
That depends on the game, devs can chose. Something like recent Ubisoft games requires the launcher, uPlay and Denuvo.
While something like Phoenix Point is promised to only require the launcher for initial installation (one dev even said that you would be able to move that installation from one Windows installation to another.