Spells that force your opponent to mill half their library are common for mill decks, but for it to be an additional cost isn't quite so common
As for this deck's commander, popular choices include similarly controlling leaders like the Grand Arbiter or more aggressive creatures that seek to put beleaguered opponents out of their misery quickly, like Harbin, Vanguard AviatorHarbin, Vanguard Aviator, or Raffine, Scheming SeerRaffine, Scheming Seer
An Eshki, Temur's Roar deck is best built as a Stompy deck (a deck that plays a ton of creatures with high power)
Strong accelerants
In Rofelloss stead, we have Marwyn, the Nurturer, which is a great choice for your Elf kindred Commander decks, either as your commander or as part of the 99