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.