I ended up building a small open-source tool that:

computes FIFO cost basis locally

exports TurboTax Online–compatible Raw transaction activity CSVs

includes a preflight validator to catch errors before upload

It’s local-first (no hosted service, no data upload) and currently supports Coinbase, Kraken, and Binance US.

Sharing in case it’s useful to anyone else dealing with the same mess: https://github.com/metalusmonk/crypto-pnl-tool

Happy to answer questions or improve it if people find it useful.