I will just ask my bank upon transferring large amount from Coinbase to buy a house. For sure, I do not qualify for a mortgage. I live in the US and Coinbase is base in San Francisco, CA. Therefore, I am sure Coinbase will certainly send my withdraw reports to the IRS, and I just need to file my tax. I do not think my bank will have a problem with me withdrawing my funds that is base in the US soil, but I just need to show my bank and file my tax to the IRS how I got the funds in my Coinbase bitcoin wallet in the first place.
Before you do anything, you need to talk to all parties about this. Talk to coinbase, talk to your bank, ask them about tax, talk to the IRS as a general enquiry to find out how to do it. Don't assume anything.
$600,000 is a lot to withdraw. Can you possibly buy a cheaper house so you are not withdrawing so much? Or perhaps delay buying the house for a year so you can repair your credit score, and just get an ordinary job, so you qualify for a mortgage? That way you can then withdraw slowly from Coinbase to repay the mortgage.