Withdrawing Tax Questions

Hi, I don't want to convert my Bitcoin to fiat now but will probably need to do it in a couple years to buy a house. I would like to understand my tax implications in doing so. I am not a US citizens, and by the time I sell my bitcoin I will not even be living in the US. However, I will be doing it using US exchanges (Coinbase, Cex) and will withdraw into a US bank account.

I need to mention, I've been purchasing and spending bitcoin here and there throughout last 5 years, and I've done it from within different countries since I've been traveling a lot, and from different exchanges to which I don't have access anymore. As for US, while being in US I bought somewhere around $1.5k worth of bitcoin. Long story short there is no way to tell the price difference between all the times when I bought the bitcoin and when I'm selling it, to effectively calculate the "gain".

That being said my main questions are:

1. do I need to pay anything in taxes in US given the situation?
2. if so, how much do I need to pay, considering I can't prove what is my real "gain" is? (other than tracking dates of receiving funds in the wallets and matching them to the price of BTC at the time). I fear that for the lack of evidence to prove otherwise, all my sold bitcoin would be considered to be a gain, regardless of the actual price I bought it for.
3. if I do need to pay in US... are there better ways to convert my bitcoin into fiat? It just doesn't seem fair that I need to pay large sum to US given that I've collected almost all my bitcoin prior to moving to US, and I will be selling it when I'm not even living there anymore.

I will also add that I don't have many options in terms of banking since I'm a Russian citizen which essentially means I have no home country and no home banking, so I'll have to stick to US banking until I find another residence.