Did i messed up my capital gains taxes?

I have been buying Bitcoin since 2017 in small and ocasionaly larger amounts, never sold any. My stack grew over time more than I ever expected.

I always filed my taxes but never reported these buys as investments. If I sold now it would be a large sum in my account. Can I declare retroactivly that I have been buying for 8 years?

I have hundreds of transactions from 10 to 5000 dollars, across many years and exchanges. I used a couple excnahges and latetly only robosats p2p. I keep an Excel tracker but it is editable and not hard evidence. I never downloaded recipts though the data might be in exchange records but getting it would be a pain.

What do people do? Sell slowly via p2p to avoid a big tax event? Sell all and pay? How is cost basis calculated with so many buys? I know a tax advisor is the smart move but I am sure someone here has gone trough this.