Hiya friend - seems like folks above already got you lots of helpful answers! As noted, the cosmetic items in the current Pre-Order Packs rotate on a monthly basis, and they are never available again once they're gone. Purchasing a Pre-Order Pack will enable you to see the relevant add-ons in future months, if you'd like to grab them on an individual basis!
In addition, the work that goes into creating these items is definitely not "thrown away" - the pets and mounts you see will have variants in-game as NPCs for you to fight/befriend, the costumes/accessories can be repurposed as NPC armor, the freeholds can take on a different appearance to fill out towns, and more - think of these cosmetics as variants for things we would need to make to bring the world of Verra to life either way!
In terms of "similar aesthetics" being available in the future, I would definitely recommend keeping an eye out for both future cosmetic sets that might suit your tastes better, or see ...
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