2 resources from Tackle we point people to, and the questions each answers.
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Exactly the case you are deciding about: one person covering more than one cloud marketplace. She explains what the second and third listing cost her in effort and what they returned, which is the real answer to whether AWS is enough.
The benchmark dataset for marketplace and co-sell: multi cloud sellers at 29% of revenue through marketplaces versus 3% for single cloud, and 92% of companies saying cloud partner attention is the hard part.