Ok so someone gives op 100btc which are now worth $600,000. 6 months for construction is very quick, but 6 months for construction and then being able to pay back 100btc is super quick. What happens if BTC is worth $20,000 each in 6 months? How would you be able to pay back $2m?

I think that this has too much risk. Look at the price of Bitcoin now, then look at the price of Bitcoin 6 months ago or 12 months ago (maybe there is a construction delay) it would not be possible for OP to pay back the full 100 btc any time in the last year as the price has risen too high too quickly.