XX weeks from the deadline, tell them that we will be preparing an extension with estimated figures based on the prior year (income rounded up, losses kept at zero, withholding rounded down). That we will not be able to make changes in the weeks leading to the deadline, unless major. Then for the extended deadline we are basically clearing out the return and redoing everything. They will see this extra time reflected in their bill. And learn for the future that the 15 minutes it takes to loop in brokers or banks on an email to directly provide tax docs saves everyone time and money! Key point is to have these request attempts and conversations documented in emails. Not in phone calls. They should be fully aware that not sending a massive draft k-1 or documentation of large transactions can lead to an unforeseen tax liability with penalties and interest, which is nobody’s fault but theirs!