I've completed thousands of tax returns and I have to say, dealing with cryptocurrency transactions is somewhat onerous, especially when compared to stock brokers who send clients a 1099-B with reported cost basis.
Once you start mixing in spending on goods and services, altcoin trades and several different exchanges --
some of whom don't provide adequate records or if they do, in really bad formatting -- it can become overwhelming. Coinbase actually has one of the better formats for filled order history that I've seen.
Do you not just report one figure for capital gains?
We do that here (afaicr), it's why I don't go through my history on every exchange and instead just tally up the assets I have at the end of the year.
Gains = Assets from this year (at close) - earnings - assets from last year (at close)? Trouble comes when you have to try to come up with the fiat value for what all your gains are at the time you sold holding for them but generally a monthly average should suffice if you can remember for the large trades (For the small ones I wouldn't worry, a best quess will suffice or if you feel paranoid you can always go with the highest price of the year (or 75% of it) if you want to cover yourself.
I made a thread recently about portfolio managers and blockfolio does pretty well at tracking what you trade when you import data from an API (but it keeps updating rates after a trade is complete as it stores the fiat rate) but it's something to consider if you want a more accurate overview (other sites must do the same too)
make sure the keys are set to read only though