As you'll be playing progressively larger creatures, this will ideally trigger each time you cast a creature
At first Angelsong or Glimpse the Sun God felt like reasonable options: you often only need to use Fog effects a few times, and X spells shouldnt be affected by Commander tax as harshly
Just be sure you have some diversity of permanents, fetch lands, etc., so you can trigger Delirium
War of the Spark Mythic Edition is limited to only 12,000 units (Limit 2 per person)
Casting X spells of any kind, will make 0/0 Hydra creature tokens, and that's the real value here
When it comes to Vivid's strength, you have to ask yourself how good an effect is going to be given N=2, as this is going to be typical for both Constructed and Limited