BTC Selling Tax Question - Calculate by average cost or by chosen lot when selling?

My average cost is $31,100. I have bought all over the place, from $19K all the way up to $62K.

When I sell, which won't be for another 10-15 years (if that, I might keep forever), how do I calculate how much tax I owe? Can I simply calculate it by my average cost or do I need to select certain lots and calculate from those basis costs?

I have records of all of my purchases.

Honestly, I have never sold any BTC I bought so I don't even know what the IRS form looks like.