Where can you find dependable partners for large-scale training data? [P]

The project is growing, and we are trying partners capable of delivering big datasets on a steady basis, potentially millions of samples, backed by strong quality checks. I have looked into the major annotation providers, but their websites make it difficult to separate the ones that handle real volume from the ones with strong marketing.

Here is what I want to understand:

Which providers have handled high-volume data collection or labelling for you, and would you hire them again?

What did you do to test quality before signing a large contract? A trial batch, a sample review, or something different?

Which warning signs did you learn to catch early, like late deliveries, uneven labelling, or vague sourcing and consent practices?

Does it make more sense to rely on one major vendor or to divide the work among a few smaller teams?

I am more interested in first-hand experience than lists of vendors. Once we decide, I will post our choice in case it is useful to anyone.