Who would buy my bitcoin? (Hypothetical question)
A lot of exchangers have such liquidity but you will encounter issues with withdrawal limits depending on your verification level. Also I don't think you can expect transfers of that magnitude to happen at once - they will likely chunk it out by agreeing it with you.

As for the tax, it all depends where you live as I believe you will be taxed on income.

We all wish we could go back in time - however you have to look to the future, what tells you 1 Bitcoin won't be worth $100,000 in 5 years time?