Does Voodoom need extra coverage?
It certainly does. Over the course of two generations, a lot of Pokemon have emerged that can wipe Voodoom of the face of the Earth without as much as a second thought. The introduction of the Fairy type in general in Gen 6 is a massive problem, as his STAB moves are now rendered near useless against them. And the introduction of the Tapus in Gen 7 sent Voodoom even further down the hole of unviability. For Voodoom to be sufficient as an offensive Pokemon in the current metagame, it needs coverage options to be able to beat these new threats. Otherwise, a opponent can switch in a Pokemon such as Tapu Fini and force Voodoom to switch, viably stopping Voodoom as an offensive threat to the team. While we certainly want specific checks and counters to remain mostly intact, such as Unaware Clefable, it is essential that Voodoom gain the ability to beat some of its current checks for it to be successful in the current metagame. Giving Voodoom at least one new coverage move (preferably Thunderbolt) allows Voodoom to fight a new number of these, and also gives it an element of unpredictability.
2.Does Voodoom need this extra unpredictability and versatility?
Think of a prevelent metagame in either OU or CAP. They are likely to have one of two things: massive bulk or use on a stall team (such as Toxapex, Chansey, or Dugtrio), or, more importantly, options. Many, many major threats in these two tiers almost always have multiple sets, numerous coverage moves, and the ability to use an array of items. Does this make them unpredictable? Certainly. Does it expand their abilities and increase their viability as a Pokemon? Absolutely. While I heavily doubt that Voodoom will every reach the level of stuff like Tapu Lele or Pheromosa, having only three usable offensive moves for an offensive Pokemon is extremely dampening of its ability. Without unpredictability, Voodoom could end up like Cawmodore: a pokemon so linear in its strategy that it becomes incredibly predictable and always loses to an unchanging set of Pokemon. Either way, Voodoom needs an element of unpredictability, even if just a slight element, to be able to execute the role that we have given it.
As for the choice of coverage, since I have yet to mention it, I'm personally sticking with the choice of Thunderbolt. I see it as the middle option, as strong Poison Coverage in the form of Sludge Bomb or Sludge Wave is extremely powerful, and it hits far too many things strongly for it not too be considered "broken". On the contrary, Flash Cannon does a somewhat similar function, but far worse. As snake mentions, Flash Cannon causes far too much overlap with existing STAB or coverage moves, essentially making it a niche option. Why I like Thunderbolt is that it functions similar to Psychic, as they both hits similar threats, are blocked by a certain type, and have overall pretty great neutral coverage. But each one hits certain things better than the other.