These are somewhat mutually exclusive.
The best deals are capped at fairly low quantities, so to buy gems the cheapest, you need to buy the best deals over time.
If you prefer speed, you’ll need to buy less good deals to get them sooner.
No, and many of the offers aren’t predictable in advance, but the recurring ones during Challenge Events, Atlantis, Valhalla, and other monthly events normally repeat each month.
Those are detailed here:
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📯 Offers – Season 3 (Valhalla) (this has the Atlantis and some other offers listed for comparison too)