ok I never used poison in gen 8, but I'll try and give some general recommendations.
You ever did dmg calcs with the Nidos while using special moves? They do insane damage. In usual circumstances, max Atk Eq and Jab do less than mas Spa EP and Wave, all thanks too sheer force. Just change Eq to EP, Jab to Wave, and Ice Punch to Ice Beam, and maybe consider switching Helm to Sludge. If you really want this thing to set hazards, you could also swap out one of Sludge Wave or Ice Beam for Spikes. Other than that, it should be good.
For Drapion, I recommend a SD set. Crobat and Salazzle got the speed anyways, and Drapion's power with SD could be insane. Slap on Knock Off, Poison Jab, and either EQ or Aqua Tail, and it should be good. Poison Jab is preferred over Cross Poison because the Crit rate is too unreliable. Knock Off's item removal makes it generally better than Crunch.
Heavy Duty Boots on Crobat. If you plan to Defog with it, you need to make sure it doesn't die from the hazards first. Maybe also put Roost over Cross Poison for longevity.
This Rose set that you have looks like a great Specs user (this is coming from a person with -2 experience using Rose)
Slowking Galar is an interesting mon, and I'm not surprised that not a lot of people play it correctly. Firstly, if you ever want to put leftovers on a poison mon, slap yourself in the face and put Black Sludge instead. It only benefiting poison types means that, if tricked to a non-poison opponent, it will hinder them instead of helping them. Secondly, the best Slowking-G sets are actually Assault Vest, to make the most of its great bulk. As for set, Sludge Bomb is recommended for Slowking's relatively slow playstyle coupled with possible poison chip. Next, choose one of Psychic, Psyshock, and Future Sight, depending if you want reliable damage, the ability to hit special walls, or the ability to synergize with other attackers. Flamethrower is great for the Steel matchup, something only Salazzle enjoys, and Scald allows it to hit ground types while also providing possible burn chip. Using Slowking takes practice, so don't get frustrated when things don't go as great at first!
Finally, the Salazzle set looks fine. I do reccomend something more like Substitute over Dragon Pulse and Black Sludge to bolster Sub, but this current set looks fine.
I'm a little shocked Nihilego isn't on this team. A monstrous special sweeper that can sweep even slightly chipped teams, Nihi is a great asset to poison teams. Poison also gives an opportunity for Toxapex and Amoonguss to wreak defensive havoc. If you want more offensive options, Gengar is a great choice, as its ferocious Ghost STAB and great speed tier provide insane attacking threats. Finally, Nidoking serves to be a more offensive option over Nidoqueen.
I do think Nidoqueen and Roserade are weak links here. No, not Slowking-G, it does take careful play, but the potential payoff is immense. If you do end up choosing King over Queen, just let Nidoking play offense; don't bother with hazards.
Also, check out smogon.com's analyses. They are way more professional than whatever I can write about, and maybe you can find even more suggestions in there.
Good luck climbing ladder!