Document how you came to have the stuff in the first place. First to defend against potential government inquiry; and second to establish that you held the asset more than one year, so you qualify for long-term capital gains rate.

I wouldn't sell it privately all at once, if you can avoid it.

If you can prove you held it more than a year, you should pay the long-term capital gains tax rate, which is fairly low. You'll keep most of it.

Be extremely wary of "the lottery winner's curse"

A huge windfall often goes very badly. People don't change their financial habits, burn through their winnings shockingly fast, overspend it, and wind up deep in debt. At the end of the crazy train, their lives end up worse.

That wasn't your question, but you'll do better if you're on guard for that, with good planning and a desire to invest it in things which give you deferred income in the future. That's the cooler thing, when your investments mean you don't have to go to work!

Not for everyone: Donate to charity!

I don't mean donate ALL of it to charity. But feel free.

If you hold a security more than one year, and donate it to charity, you get a tax deduction for the appreciated value (even though the security didn't actually cost you that). (link) Do not convert the BTC to cash then donate the cash. Donate it as BTC.

Your tax deduction works against your highest tax bracket. If you are paying in a 28% tax bracket (your next $100 of income has $28 tax), then for every $100 of charitable donation, you get $28 back on Federal. It does the same to state tax, and you also avoid the 10-15% capital gains tax because you didn't sell the securities. Do your 1040 both ways and note the difference.*****

Your charitable deduction of appreciated securities is capped at 30% of AGI. Any excess will carryover and becomes a tax deduction for the next year, and it can carryover for several years. **

Use a donor-advised fund.

If you have are donating more than $5000, you don't need to search for a charity that will take Bitcoin, and you also don't need to pick a charity now. Instead, open a special type of giving account called a Donor-Advised Fund. The DAF, itself, is a charity. It specializes in accepting complex donations and liquidating them into cash. The cash credits to your giving account. You take the tax deduction in the year you give to the DAF. Then, when you want to give to a charity, you tell the DAF to donate on your behalf***.

You can tell them to give on your behalf anonymously, or merely conceal your address so you don't get the endless charity junk mail. The DAF lets you hold the money in index funds, so your "charity nest egg" can grow with the market. Mine has more than doubled thanks to the market. This money is no longer yours at this point; you can't give it back to yourself, only to licensed charities.

The Fidelity Donor Advised Fund makes a big thing of taking Bitcoin, and I really like them. ****

I love my DAF, and it has been a charitable-giving workhorse. It turns you into a philanthropist, and that changes you life in ways I cannot describe. Certainly makes me more level-headed about money. Lottery winner syndrome is just not a risk for me (partly because I'm now on the board of charities, and oversee an endowment.)

Donating generally will reduce suspicion (criminals don't do that), but donating to a DAF even moreso. Since the DAF would have to return ill-gotten gains, they're involved. Their lawyers will back you up. The prosecutor is up against a billion dollar corporation instead of just you. With Fidelity particularly, Bitcoin is a crusade for them, and their lawyers know how to defend Bitcoin. A Fidelity DAF is a good play for that reason alone IMO.

** The gory details: Presumably you are donating to regular charities or a Donor Advised Fund, and these are "50% limit organizations". Since it's capital gains, you have a 30% limit. If your donation is more than 30% of AGI, or if you have carryover from last year, you use Worksheet 2 in Publication 526. You plug your donations into line 4, then the worksheet grinds through all the math and shows what part you deduct this year and what part you carryover to the next year.

*** I specifically asked managers at two DAFs whether they were OK with someone donating a complex asset to the DAF, and immediately giving the entire cash amount to a charity. The DAF doesn't get any fees if you do that. They said not only are they OK with it, most of their donors do exactly that and most DAF accounts are empty. They make it on the 0.6% a year custodial fee on the other accounts, and charitable giving to them.

Mind you, you can only donate to 501C3 type charities, what IRS calls "50% limit organizations". This actually protects you from donating to organizations who lie about their status.

**** I'm not with Fidelity, but I am a satisfied DAF customer. The DAF funds its overhead by deducting 0.6% per year from your giving account. If you invest the funds in a mutual fund within the DAF, that investment pays the 0.08% to 1.5% expense ratio of the fund. I can live with that.

***** I just Excel'd the value of donating $100 of appreciated security instead of taking it as capital gains income. 28% Fed tax, 15% Fed cap gains, 8% state tax on both. Take the $100 as income, pay $23 in cap gains tax. Donate $100 in securities, the $23 tax goes away since you didn't sell it. Really. The $100 charitable deduction offsets $100 in income, also saving you $36 in regular income tax. Net tax savings $59. However you lost the $100! So you are net $41 poorer. It costs you $41 to donate $100 to charity. This gets better in higher brackets.