Cool. I've been thinking about this recently and long-term CGT means it's probably not a good idea to cash everything out in one go.
This would only be a good thing if they let you still hodl your bitcoin keys.
If not, then no, not a good thing.
Pretty sure that you would indeed have to custody your crypto with them.... or at very least sign it off to a contract.
..... and, that leads that to the other main thing to be wary of.... and that is the liquidation risk, you would not want super thin LTV with a loan like this... and you would want/need a a lenient window to post collateral.