Hi there! Welcome to the competitive aspect of Pokemon cards!
There's lots of cards that improve consistency. Even cards that don't always immediately prove helpful early game like Super Rod can increase consistency late game. If you want to get find out which cards are good enough staples to be included in the majority of decks, go check out the Deck Garage. You should be able to get a feel for which cards can provide consistency by looking at a variety of decks. Some cards off the top of my head that Yveltal/Zoroark can use to increase consistency are Max Elixir and Captivating Pokepuff. Max Elixir can accelerate Energy onto Yveltal EX, making Evil Ball faster and powerful and Captivating Pokepuff can increase the damage output for Zoroark. It can also make use of some of the cards you suggested like Sycamore, Mail, Ultra Ball and Acro Bike.
Hope this helps!
