> - Call their API through ipv6, because they may not yet have a proper, ipv6 subnet-based rate limiter.

Nice tip!!

> -- From a purely practical perspective, start with a baremetal transaction-isolation-less database like Cassandra/ScyllaDB. Don't rely on googling "postgres vs mongodb" or "sql vs nosql", those articles will all end in "YMMV". What you really need is massive IOPS, and a multi-node ring-based index with ScyllaDB will achieve that easily. Or just use MongoDB on one machine if you're not in hurry.

Somewhat ironically Elasticsearch would probably work really well for this too (just make sure your elasticsearch isn't open to the world on the internet!).