That doesn't really tackle my point regarding that you don't know what your gains are, because you don't know which coins you are exactly selling. If they are all fractional-mined at different values, you are getting profits or losses depending on WHEN they were mined. There is no way to track that information.
It would be pretty easy to write a script, using the blockchain API to iterate all the mining receive transactions during the tax year on the receiving address(es) and to get a historical exchange values from the same API. The databases exist for free of charge, and you most likely have a computer that could run such a script.
please re-read my posts regarding the question remaining, how do you know which cashed out BTC, are from which exact mined BTC.
It helps zero to know, you received x btc at x price.
You need to know, that xyz bitcoin was recieved at xx price, and those same xyz bitcoin fraction was sold at yy price.
That is a non-issue. Most (as in 99.99999999999999999999999%) of stock trades are exactly the same way.
You buy 100 shares of Apple, then 20, then 50, then 30. Later you sell 60 which 60 are sold. Broker report simply shows bought and sold, they don't give you assigned share numbers.
If you can't track individual shares (which you actually could with Bitcoin using coin control) it is FIFO. The IRS is fine with that as long as it is consistent. So in the Apple example above the 60 shares would come from the 20 share buy and 40 of the 50 share buy.