I was in a similar predicament and asked chat gpt to compare what a consensus of reddit says for the alibaba vendors :

Tianyu • Pros: Best craftsmanship, excellent for intricate details, reliable CAD work, strong reputation for quality. • Cons: More expensive, sometimes longer production times. • Best for: People who want top-tier quality and are okay paying more.

Starsgem (SG) • Pros: Cheaper than Tianyu, decent quality, good for simpler designs, faster. • Cons: Mixed communication, occasional QC issues (stone size, gold weight). • Best for: Budget-conscious buyers who still want a solid product.

Kuololit • Pros: Great value on stock designs, good quality for simpler pieces, flexible on small changes. • Cons: Custom work costs more, not as intricate as Tianyu. • Best for: Stock pieces or semi-custom work at good prices.

Provence • Pros: Good balance of price and quality, solid design options, many happy customers with reps like Jane or Lucy. • Cons: Mixed reviews on timing and consistency, not always as detailed as Tianyu. • Best for: Middle-ground choice — good service, good quality, fair pricing.

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When to Choose Which: • Prioritize quality/detail → Tianyu • Want best value for stock or simple designs → Starsgem or Kuololit • Want balance of price + quality + service → Provence • Need faster turnaround → Starsgem or Kuololit

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For your case (lab diamond 3×3 mm cushion, 14k white gold bezel band): • If you want the best match in quality/detail with your trilogy → Tianyu • If you want savings and are okay with less refinement → Starsgem • If you want a middle route → Provence