Each instance of cascade triggers and resolves separately. The spell you cast due to the first cascade ability will go on the stack on top of the second cascade ability. That spell will resolve before you exile cards for the second cascade ability.

Maelstrom Wanderer gives itself haste.

No matter what spell you cast with the first cascade trigger (or with any cascade triggers that result from casting that spell), the second cascade trigger will look for a spell with mana value less than Maelstrom Wanderer's mana value of 8.