holocaust

How did he not cause the Holocaust? Is he responsible for everything and why did he let it happen? And in general, why does he let bad things happen (like diseases and crimes)?

You opened Pandora's box here. In my opinion, he did not do the Holocaust and it is not true that he is responsible for everything. People did the Holocaust. Regarding evil in the world, search the site, there are many discussions about it.

A detailed discussion of my trilogy that will be published soon from MS.

Why not say that the first assumption, God is good, is not necessary?
God has defined what is good for us and expects us to act according to it, why is it reasonable to assume that he also acts according to it?

As I recall, in the questions about Euthyphro's dilemma, you claim that God chose the good arbitrarily, but once he chose, we are obligated to obey him. If it is arbitrary from his perspective, then it is more reasonable to say that he himself is not obligated to it, but only created it “for us”.

Nothing is necessary, but it is still a very reasonable assumption. He also tells us that he is good, and that is how it is described in the Bible. He cares for us, even if we have questions about some of his actions.
In Euthyphro's Dilemma I don't remember writing that good is arbitrary. On the contrary, I argue that it is a result of the world being subject to it. In this world, good is defined and cannot be changed. But God is the one who decided to create the world as it is, and He could have done it differently (then there would have been a good name that was defined differently).

What is the source of this in the Bible? It doesn't come to mind.

See Psalms 9, as well as the Lord, the Merciful and Kind, and more.