Consistency is the key to winning ladder. No deck will win every time, but decks that can set up their win condition reliably and quickly tend to win more than those that can't. I myself am fairly new to Pokemon TCG Online but I managed to get the second Alolan Ninetales GX up for grabs before the ladder was reset in tandem with the rotation by playing in Standard half the time and Theme the other half of the time. If you want to go the Theme route, Leaf Charge and Mach Strike are probably your best bets, but most of the decks from Sun and Moon onward can probably compete with each other.
But if you really want to go Standard and don't want to spend money, I've found some great budget-friendly options to help the consistency of any deck and have been using them as a "skeleton" of sorts for my own Standard decks. Below is a list of cards I've found useful for this purpose, what they can do for your deck, and how to get them:
Strike and Run Dunsparce - This is extremely good for first-turn plays. For only one colorless energy you can place up to three Basic Pokemon from your deck straight onto the bench, and then you have the option of switching it out for free. It thins your deck and sets up your next plays. It can also cause paralysis with its second move (also only one colorless energy) if you're lucky, which adds some stall power while you're pumping up your win condition with energy. One copy is in Leaf Charge and Hydro Fury, so it's only 1000 coins to get two.
Call for Family Mawile - This is Dunsparce's sister in the domain of set-up Pokemon. She can search for two Basic Pokemon with only one colorless energy. It's not quite as good as Dunsparce's three, but she compensates by having more health and dealing more damage with her second attack. She's also easier to get, since Destruction Fang comes with two.
Instruct Oranguru - If you're low on cards, have no fear. Once per turn you can draw until you have three as long as it's on the field, making your deck just a little bit faster. Since it's Basic, Dunsparce and Mawile can search it, adding to the consistency. It can also KO opponents mid-to-late game with its attack, Psychic, which does 60 damage plus another 20 for every energy on the defending Pokemon. Most big attackers need three or four energy, which means 120 or 140 damage on average, which OHKO's a lot of threats and even two-shots most GX Pokemon. The attack requiring three colorless energy means the attack can work regardless of your deck's color too. Leaf Charge, Destruction Fang, and the Incineroar deck whose name escapes me all have one. You could (and I did) get 3 of this card by getting one of each deck, though you'd probably only want to run 1 anyway since it is not good as an opener.
Excavate Sableye - This is a whole lot better than I first thought. Its ability lets you look at the top card of your deck, and then discard it if you don't like it. This can help thin out your deck by milling the Dunsparce and Mawile you don't need anymore after you've already filled your bench. Or if you need a second energy in the discard to make full use of Energy Retrieval, you can do that too. And when you choose not to discard, that means you have an additional resource to consider when responding to your opponents' plays. Knowing what you're going to draw next turn, almost every turn, gives you a considerable knowledge advantage over your opponent. It's an uncommon card in Celestial Storm, so while most people would have to rely on luck to pull one, according to the OP you apparently don't have that problem.
Cynthia - Most people say this is the best draw card in Standard format as of now. Thankfully Mach Strike comes with two and Twilight Rogue comes with one, so you can get 3-4 copies very easily.
Guzma - A free switch is good. Forcing your opponent to switch without their consent is even better. Doing both with one card can win you the game. The top-tier decks run 3-4, and it may be unreasonable to obtain that amount, but one copy is present in Leaf Charge. And so is Escape Rope for that matter, which is not as good as Guzma but compensates by being an item you can play in conjunction with other trainers. Escape Rope can certainly take Guzma's place in budget-friendly decks.
Tate and Liza - You either have a slightly worse Cynthia or a free switch by using this card. Both have their uses. Like Dunsparce, Leaf Charge and Hydro Fury come with one each.
Ball trainer cards - You need to get Pokemon out early. Ball cards get them. Ball cards are also items, so they can be used multiple times per turn. Nest Ball is the best one for Basic Pokemon early-game since there's no RNG involved. Timer Ball is probably the best one for evolutions; two coin flips, one evolution for each heads. Ultra Ball also has no RNG and can search any pokemon, but requires two discards. Still worth considering despite that though, in part because Ultra Ball can get your hand small enough for Oranguru. Balls are also extremely common in Theme decks to the point where basically all of them have two Nest Balls plus at last one other kind of Ball; you probably already have more Balls than you will ever need.
Lady - You also need Energy early. Lady searches four basic ones. This thins out your deck and gives you resources for several turns. She conflicts with Oranguru to the point you will almost never use Lady and Instruct in the same turn, but that's peanuts compared to the upside. Professor's Letter was rotated out, so for energy searching you have to rely on the more powerful Supporter variant. And again... Leaf Charge and Hydro Fury have one copy each.
I hope this helps you construct a Standard deck for the new format.