Despite a hefty investment of five mana, one mana a turn to turn dead creatures into Dollstructs is a competitive rate
Auras are the best way to boost Sigardas power since a Selesnya commander has access to all the best enchantress cards, including Sythis, Harvest's Hand and other enchantresses to draw cards and spells with umbra armor like Snake Umbra to protect Sigarda from those board wipes
Scourge of the Throne, meanwhile, is best used in dedicated Dragon tribal decks where it can benefit from cost-reduction effects or tribal synergy, and the extra combat steps and +1/+1 counters from the Dethrone ability are merely bonuses that push strong Dragon decks over the top
Selvala's last ability looks at the creatures you control as it resolves to determine how much mana of the chosen color to add
What Im saying is to take my advice with a grain of salt if youre looking to figure out which venture card is the most competitively viable
However, it requires you to cast noncreature spells or cards that provide oil counters in order to enable its tap damage