Wrong, the evidence is not almost entirely testimony. In addition to witness testimony, which isn't so easily handwaved away, and which comes from victims, bystanders, and perpetrators, a great deal of the evidence is in the form of documents. And there have been forensic studies of both gassing and shooting sites, much as deniers ignore these. What there is very little evidence for, on the other hand, is the twin denier gambits that the witness testimony and incriminating paper trails were on the one hand coerced and on the other manipulated or forged. We are still waiting, for example, for the promised deconstruction of OSR 24. And waiting.
Let me introduce you to, to take two examples of those who have relied heavily and successfully on documents, Mr Hilberg and Mr Longerich. But there are many other scholars I could name whose careful work in archives reconstructs the decision-making and impact of the genocide. Roberto Muehlenkamp has written extensively on forensic studies of the genocide at Holocaust Controversies, to take another example of what you are ignoring.
Unhappily for your side, these various types of evidence tell very similar stories in the case of the genocide. There is an immense volume of this evidence, and it fits well together. It comes from many different sources and streams. That is why you are reduced to your core values and beliefs, expressed here as your common sense of what is implausible or not. Your point of view is essentially worthless and laughable for what it ignores. And for most deniers "it's implausible" is a polite coverup for antisemitism and Nazi style ideology. I don't know you or your motivations. Just saying.
I am sure you are aware that Irene Zisblatt's claims are not what is at question here. In fact, they are not taken seriously, as evidence in favor of death camps, by scholars who research the extermination of the Jews, witness Joachim Neander's article on her book. Again, your raising Zisblatt only shows your ignorance of the field or your mendacity, given that she has been "debunked" rather noisily by "believers." You may find this implausible but it is a fact: scholars of the Holocaust do not treat all testimony the same and weed out bad testimonies, which are as much a part of the sourcing for the Holocaust as they are, say, for major wars.
Given your attachment to uninformed common sense and your dismissal of major types of evidence for the extermination of the Jews, to be consistent, you are now going to have to throw out a great deal of human history, including, for example, events like Stalin's purges and show trials and the mass starvation in the Ukraine, which rely on evidence not to your suiting. In fact, most historical writing uses documents and, where they can find them, historians are eager to consult testimonies in various formats.