Missing 1 BTC. How to handle 8949?

Long story short, bought ~4.5 BTC over the last ten years. During the early years, I sent BTC to other exchanges to buy other alt coins, etc. At that point, this was not a taxable event and you could only buy altcoins with BTC.

In 2023 I decided to sell all my BTC (and later rebuy) just to be done with this never ending cycle of uncertainty. So starting in 2024 tax year, I will all be easily kept track of.

When I sold my BTC, it was 3.47 BTC, so I am missing roughly 1 BTC.

How would you recommend I handle this tax wise? I do FIFO. If I left the most recent buys out, that feels like an obvious thing not to do since they are the most recent.

Just trying to make the best of this situation and figure out the best way how to fill out my 8949. And before someone suggests using crypto tax software, that won’t work. These transactions go back 10 years and there was way too much back and forth to capture it all.