I checked again, and you're right. Interactive Brokers is the only platform I still infrequently use but it does have a list of pre-filled tax forms available for download. Worksheet 8949 is also pre-filled, but this is probably where one would need to submit a revised version if previous cost-basis amounts need to be adjusted as in the specific edge cases we're talking about above.

Coinbase seems to have sent 1099-K documents to eligible customers which isn't as detailed as 1099-Bs since I believe the Ks only report gross transaction volumes over $20K[1].

[1] https://www.irs.gov/businesses/understanding-your-1099-k