IRS assumes like  you bought BTC at zero if you can't provide a data?  That might be too much of a greed on their side but where did you get the information?

It could be because its easier that way than confiscating your BTC. So what would you rather do, provide the proof of how and when you got the coins or simply give it to them? Its kind of a  hostage. Funny but I want to read some other's opinion.

All countries have different legislation, and if you plan to invest that kind of money in cryptocurrency, then you need to study tax issues or consult with a tax advisor in your country.In many European countries, as well as in the USA, exchanges provide an extract on the purchase of cryptocurrencies and help to generate a tax return through their software. If you need purchase documents, then take care of them before buying.

Preface: I am not a tax expert.

Here are some relevant parts from the IRS website: https://www.irs.gov/individuals/international-taxpayers/frequently-asked-questions-on-virtual-currency-transactions
Read Question 27 about peer to peer transactions and Question 46 about maintaining records.

If you are trading peer to peer, then you simply need to keep your own records which are comprehensive enough to satisfy the IRS.

If the IRS coming knocking after you sell an asset and you cannot reasonably prove the cost basis at which you acquired that asset, then they will assign a cost basis of zero. This is the same for all assets, bitcoin included.

How would receiving the bitcoin as a gift work? I think gifts are exempt to a certain point.

Due to needing to keep records, it would probably be recommended that despite dealing with someone in person, you should probably still go through a peer to peer exchange, so at least you can keep a record of it. Print that record that the exchange provides, and submit that to the tax authority. What they accept is up to them, but I should think that would probably be enough.

You could potentially even include in your reference that you sent from your bank, the amount of Bitcoin bought, and at what price. Then, your bank would keep that reference, and you could potentially prove it that way. I mean if you're in doubt, contact them directly, and see what evidence they would require in this particular instance.

The answer to this is given in the same link I provided above: https://www.irs.gov/individuals/international-taxpayers/frequently-asked-questions-on-virtual-currency-transactions
Questions 31 and 32 deal with receiving bitcoin as a gift. You have no tax liability on bitcoin received a gift until you do something with that bitcoin. Your cost basis for the gift will be the same as the cost basis of the person who gifted it to you. If you can't prove what their basis is, then your basis will be zero.

Putting bitcoin up as collateral for a loan will have different tax implications, but I have no idea what those implications would be because I've never done this. My understanding is that it is incredibly risky, and if you get liquidated then you will end up still owing just as much tax as if you sold the bitcoin, but you'll only have a fraction of the bitcoin value in fiat in your possession.

There is no anonymous Bitcoin. If you use a good Bitcoin mixer/ tumbler service like Chipmixer, you will have better transactions for your privacy and anonymity. Bitcoin transactions are not anonymous by default. It depends on how you mix and use your inputs, how to spend them in outputs, and how to you broadcast your transaction with Tor or no Tor connection.

It is same for tax. Because to buy Bitcoin, most of people will use fiat currency or they use fiat to buy stable coin as a middle currency to buy Bitcoin. So if they have to do KYC in first step to buy stablecoin, and later they take profit of their Bitcoin to stable coin. Governments will know it and it is just matter of time and how government will make tax call.

You can not hide yourself and your asset if you do bad things for your inputs into Bitcoin, stablecoin, etc.

You can export the data from p2p and use it for tax reporting purposes there is no other way and you have to keep track of all your trades and there are some features available on the exchanges as well but if you are someone who trades for huge volume in a year then you should consult the tax advisor and get the actual solution depends on your region.

Yes you have to be able to prove your cost basis, otherwise they assume it is zero.

For example, for my 2018 taxes I just put my year's starting and finishing trading amounts on my taxes to figure out my total capital gain/loss. In 2020 IRS hit me with a letter saying I owed them $800k in taxes plus $300k in fees because Coinbase had sent them my trade volume for 2018 and the IRS took my volume as my sell volume and the cost basis as zero, and since my trade volume was obviosly many times higher than just my starting and ending trading amounts, they said I didn't account for my trades. So I had to spend 4 weeks finding as much 2018 data as I could to prove that I owed them nothing.

Which is why people should not get into trading lightly. Make sure you are tracking your trades, or just trade on one exchange, or make sure you are doing things that will be easy to add up come tax time.