With BTC there isn't a broker so I assume you simply need to document your decision but I'm not a tax lawyer so I'm not really sure.
The blockchain is the most perfect broker record you could hope for.
With coin control you could sell exactly the coins you wanted to at exact the basis you wanted to at any time.
How? as far as i know you can choose wallet to spend but btc are stored in transactions.
I can have bunch of tx in that wallet, but i cant pick the a specific one, correct?
You can if your wallet software supports coin control.