Calculators already have the energy generated by Fast moves and the energy generated by taking hits included in their DPS formula. That's why Lick and Shadow Claw have similar DPS on Gengar, even if one deals more damage but has less energy gain, and the other grants more energy but deals less damage. That's also why every Pokemon's listed DPS goes slightly down if they resist the other Pokemon's attacks. DPS might factor in energy per second, but why doesn’t anyone think Weavile is better than Gengar unless the foe is Psychic? I know it had an Ice-Type Fast move, but it does a lot faster as well.
Also, bare in mind the amount of damage Gengar deals with fast moves will be less. Which means the charge move of the Pokémon it faces will charge slower. This lets Gengar take advantage of charging fast, but charging the opponent slower, which contributes to longevity and therefore TDO.
Those same calculators did actual math to calculate that Roserade has a higher DPS than Gengar against most Fairy-types. I'm personally going to rely on that.
And it's true, Poison-type moves are not so useful for now. There hasn't been a single Raid Boss so far that specifically required the use of a Poison-type attacker. Against Grass, Moltres/Charizard/Mamoswine/Honchkrow are all better, and Fairy Legendaries were only introduced in Gen 6. But you specifically mentioned the Tapus, against which, until we know their specific movepool, Roserade is still listed as a better overall counter (except, as stated, Lele)
As far as Togekiss goes, Rampardos, Metagross, Electivire, Deoxys, Raikou, Mamoswine, Dialga, Roserade, Zapdos, Luxray, and Weavile (in a varying order depending on Togekiss' moves) are all listed as having a higher DPS than Lick/SludgeBomb Gengar.
Look, if you like Gengar, have many powered up and like using it, I'm not here to deter you from using it. It's still good against something weak to Poison. I'm just saying that Gengar is not the optimal choice for Poison, and that there are better options.