I had a similar thing happen recently. Some of the IP addresses were proxy / datacenters but many of them weren’t, which made me think it might be a botnet. And the UAs were generic, so there wasn’t anything easily-bannable.

I added fingerprinting and rate-limiting and the problem seems to have gone away. They’re trying to test a large number of accounts / credit card numbers so the best strategy is to slow them down to the point where it’s no longer worth it for them at scale.