The major problem is how good Conjure Minor Elementals is when you have lots of attacks. I have played Warlocks, and there's plenty more to them than just their spell slots. Conjure Minor Elementals should have a maximum of adding to at most 3 attacks per round to better balance it.
Maybe the Potent Dragonmark feat needs to be nerfed to providing a slot of character level/3 (round up), instead of character level/2 (round up). That still gives a level 2 slot at character level 4, or a level 3 slot at character level 8 if taken then, but it defers them getting a 4th level spell slot from the feat until character level 10; and defers a 5th level spell slot from the feat to character level 13.
In my opinion, it's a good thing to have a feat chain that can provide a light multi-classing equivalent. However, the character level/2 (round up) spell slot regained on a short rest does seem fairly powerful, given that it is faster spellcasting level slot progression than an Arcane Trickster or Eldritch Knight. So nerfing it to character level/3 (round up) seems reasonable at first glance to me.
