Well. It's great you have those decks.
You've gotten staple trainers as Cynthia (a couple in Mach strike) and Guzma (one in Blazing volcano) and more and you start a pool of pokémon that you can use in a lot of decks (Magcargo for example).
Easier way to start would be getting a couple Mach Strike to make one competitive (not meta but good) useful deck. You only need to remove useless cards and add enough of what it's important (Garchomp, Lucario, Cynthia...). This can also be done with Imperial command Theme, but I've never seen as good results as with the Mach strike modified deck.
You can visit Item exchange subforum and check 'Watto's junk yard' to ask for some trainers you miss.
Actually I would suggest playing Theme deck VS to grind a bit the ladder with any Theme you control for a while. It's boring but you can get rewards from ladder and go to tournaments to win Unlocked packs, and then trade them for the Meta cards you need.
Meta cards yes, they cost around 10-20 packs each. Tapu Lele GX, Zoroark GX, Alolan Ninetales GX (fairy), Blacephalon GX... The good is you need just 4, and then you can build different decks with the same (Zoroark GX it's in a lot of decks: Zoroark/Magcargo (=aka Zoroark control), Zoroark/Golisopod, Zoroark/Decudieye and in Expanded Zoroark/Seismitoad...). But still a good amount of packs.
I would get 1 Tapu Lele GX with your 15/18 packs.
Then try to get the cards for Malamar Toolbox, Psychic Malamar or Ultra Malamar. Probably 6-10 more packs can do it. You can play it without but, better with one Lele tho.
Also probably you can get everything for the Lost march with those packs, or Alolan Exeggutor.
Or you can go Expanded to Night march, Turbo Darkrai, Trevenant Break, Primal Groudon/Wobbufett, Drampa/Garbodor, Alolan Exeggutor too, Greninja Break, Necrozma Garbodor... Check Limitless.
You can do inverse, get the deck and with it, win packs enough to get the Lele after, so you won't boring with Theme decks. That's your choice.
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