HE was indicted by a grand jury, not a prosecutor. The prosecutor has a lot of control over that process, but one would never be able to control the timing as closely as you suggest.
I think Biden should testify and I think he should plead guilty to the charges and accept his punishment. It is clear to me he is getting treated worse by the DA because of who he is and the politics of the situation, but maybe that is a good thing, maybe people from elite families should be held to a higher standard.
It is very entertaining to me to see the same Fox News commentators who were silent about Congressmen refusing to testify against Trump under subpoena, now attack Hunter for doing the same thing.