Calculating gains and losses when spending bitcoin is incredibly impractical, especially if spending Lightning, even more if spending from a non-custodial Lightning service. You would have to figure out the cost basis for the Bitcoin at the time you opened the channel, any difference in exchange price at the time of spending, and total this up for all your dozens or hundreds of little transactions. As you say, it's a bookkeeping burden none of us are prepared or equipped to manage, even with the most sophisticated software. The more people who say fuck it the better.