While I think this impeachment trial needs to stop and the voters need to decide it, I do want to note I think executive privilege should not apply in an impeachment trial.
The President of the United States is not in a criminal trial. There is no right to avoid self-incrimination. The three branches of government are not equal. Article I actually is predominant except to the extent it cedes power to Article II or Article III. The Congress is at its most powerful when it joins with the presiding officer of the third branch of government to try the Chief Executive Officer of the nation.
To make an informed decision on his removal, the Senate needs every single relevant piece of information. That means they should be given everything including that which, under normal oversight, could not be gotten. Impeachment is an extraordinary step and executive privilege is a constructed privilege of the executive that can be used in some limited circumstances, but not every circumstance.
An impeachment trial is one of those times it should not be allowed. There may be sensitive matters that require the Senate to go behind closed doors to hear testimony and to redact testimony from the public record, but the Senators, as representatives of the several states and arbiters of the fate of the leader of the second branch of government, deserve every single piece of information available and all available testimony.
Certainly then the testimony should be prohibited for use in any criminal trial that might thereafter occur. Certainly prevent access to privileged material prior to the trial and in all other oversight circumstances. But senators, acting for their states, deserve all the information.
In a presidential impeachment trial, the powers of the constitution flow to the several states through their dual representatives to decide the future of the nation. The power to obtain the secrets of the executive that he does not want to share must flow with all the rest so that the states can make the most informed decision possible.