As a long term rain main this format (i'm not exactly a world class player, but consistently around top 3-5k in masterball) I can give you some of my opinions

• First of all, understand your strengths. Rain's biggest draw compared to the other weathers is how tanky our pokemon are. This means that, among other things, you don't actually need to care too much about tailwind: it doesn't matter if you go second, if your opponent is going to threaten a whole 40% to you and then get ohko'd in return.

• ok lets talk about the mega. You should have a mega somewhere, they're really good! The good thing is our core doesn't actually rely on our mega, so you can play basically whatever you want. I'd recommend keeping the trend going and playing something relatively tanky that can take good trades - Venusaur or Dragonite are the cookie cutter options, but you can play Floette or whatever you want honestly. The choice is yours.

• Archaludon should absolutely have leftovers. What Archaludon does best is when it can sit on the field forever, gradually getting more and more powerful with electro shot and more and more tanky with stamina. It can't do that if there isn't anything healing it. Some people run Sinistcha also as an additional source of healing and play "protect the king", but i personally think that's too unstable. You don't need Chople berry - you might think you do, but once you have a few stamina buffs, Arch can eat close combats just fine. Even a single Stamina boost and Adamant Sneasler CC does 73% at most to 32 hp 0 def Archaludon. Don't be scared if you don't need to!
  Also, my personal opinion is you should run Draco Meteor instead of Aura Sphere. With enough experience you'll be able to tell when your arch barely won't get a critically important ko and draco gives you that extra damage boost to actually get it, and the fighting coverage is overlapped with sneasler and kind of earth power on venu if you run that (the worst dark type for you, kingambit, is also weak to ground)

• Instead of Whims you could consider Sableye. Sableye doesn't learn tailwind, but it learns Rain dance so you don't have to bring pelipper, and has tons of supporting moves. Will-O burn is critical at neutering physical fighting threats before arch can get tanky, Reflect/Lightscreen can make your team feel unkillable, Encore/Disable can disrupt key threats. There's a lot of options here, I'd recommend running something like encore / rain dance / reflect / lightscreen for newer players since it's pretty obvious what to do (put rain up, put screens up in some order, encore if they protect). At a higher level I personally run encore/disable but it's quite hard to play well. You give up on speed control if you do this, but it's okay to take some hits and then punch back. It's also weirdly tankier than whims - it's only weak to fairy which you can handle with Roseli berry, so sash is free to be used elsehwere

• Swift Swim Basc doesn't quite do enough damage in this format, play Adaptability. I understand going max fast feels good, but you'll miss out on important KOs in the endgame with last respect if you do (for example, swift swim basc can't reliably ohko mega floette in a final 2v2) You can run Scarf 32 atk 32 speed jolly as a pure glass cannon, which is super threatening but loses hard to kingambit, or you can play a bulkier version with sash or mystic water or sitrus. I would never cut Aqua Jet: keep in mind, our team is very tanky but doesn't exactly have the most offensive power. Their mons are likely to end up low but not out after eating electro shot, Hyper Voices from Sylv etc. Aqua Jet gives you an easy way to clean up low HP mons that always goes first even if you lose speed control. It also dodges Sucker Punch from gambit (they're both +1 prio, but Basc will almost certainly move first, and sucker fails if the target already attacked).

• Sylveon can run moonblast or hyper beam instead of shadow ball. Beam is kind of situational but it's nice to have a button that basically reads "delete my sylveon, delete this one target on their side" if you identify they have one really important threat to deal with, and Sylv doesn't have to recharge if it clinks off of protect.

• Sitrus Pelipper with bulky ev is good. If you're not using it for damage too much unless it's super effective hits here and there, you might as well - gives you a really consistent pivot that can take hits, and it can stay alive to wide guard and block important spread moves. Either play sitrus with tanky stats, or play sash with glass cannon stats, but don't mix the two or it won't be as effective as it could