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1e6:1 is extreme. 1e3:1 is often realistic (think views to shares on social media). 18:1 is a actually a pretty good real world ratio.

If it were me, I’d just change the weights for each class in the loss function to get them more or less equal.

190m examples isn’t that many either — don’t worry about it. Compute is cheap — it’s ok if it takes more than one machine and/or more time.