1. President Trump has already said that if Bolton, Mulvaney, and others are subpoenaed to testify that he will claim executive privilege, which is his right as declared by the SCOTUS. Congress can challenge that right in the courts under certain circumstances, and no doubt the decision would have to eventually be decided by the Supreme Court. Arguments are heard on both sides, and the justices issue a verdict, but all of that takes time. Meanwhile, the Senate is on hold, doing nothing else until the trial is over. How long does this travesty continue?
2. The democrats in the House should have subpoenaed everybody they wanted to hear from, and if Trump doesn't allow it then they should have gone to court and forced the issue then. But they declined to do that, Trump is an immediate threat to our national security and our free and fair elections, blah, blah, blah. Which we now know was a complete lie, because after they passed the Articles of Impeachment, they sat on them for a month, so much for the immediate threat. My point is, the democrats have indicated through their actions that the testimony and documents were not essential to their case against Trump, they believed that they had enough to impeach him with what they already had. So fine, roll with what you already have then. It's not like they can't have another Inquiry and impeach Trump a 2nd time if they want to.
3. It is not incumbent on the Senate to accumulate more evidence against Trump. Every American has a right to a speedy trial, including President Trump, and the fact that it isn't a criminal trial doesn't matter in that regard. Due process and his civil rights should not be discarded, and I very much doubt that Chief Justice Roberts would allow that. In this country, no one is presumed guilty if they take the 5th amendment, or in this case claim executive privilege. What the democrats have done is slap together an extremely weak case that wouldn't see the light of day in a criminal proceeding.
4. Having said all of that, it is possible that Trump may allow a recorded deposition of pre-approved questions that would not reveal the inner workings of his administration or the confidential conversations and documents that should remain private. He might do that and he might not, we'll see if it even gets that far. It could be that the president's team will call for a vote to judgment or a vote to dismiss after phase 1 of the trial is over, assuming the kill-switch option is given to them. So the question of more witnesses may be mute.
But, to answer the question in the thread title: No, I don't think Bolton or anyone else should be allowed to testify. The Dems impeached Trump, and the Senate trial should be limited to the same evidence they used to reach their decision. As a result, maybe future HoR will be more likely to take a little more time and do the impeachment process with the fairness and integrity it deserves.