I do not know where you get your information, but much of it is simply false. In fact, it's just nonsense.

Here's some examples. OJ Simpson was prosecuted criminally (acquitted) but sued for wrongful death and found liable. Trump was found civilly liable for defaming Carroll, and then found liable again for repeating it. Hunter Biden paid civil penalties for tax evasion, but is now being criminally prosecuted for it anyway.

As the defendant in this case, Trump has the absolute right to not take the stand, period. He cannot be compelled to get up there and refuse to answer questions in front of the jury. The prosecution cannot even hint at Trump deciding to not testify (if that is what Trump decides).

If double jeopardy from the other fraud case applies to this case, then Trump couldn't even be prosecuted in this case. I mean, duh.

It should also be noted that the "past case" of business fraud in New York for which a judge recently found Trump civilly liable is unrelated to the criminal fraud he is charged with in the current trial. The civil case focused on falsification of Statements of Finance Condition (SFCs) to secure more favorable loans. At issue in this criminal case is the falsification of business ledgers (and so forth) to falsely recast hush money payments to porn star as business expenses (legal fees to a lawyer). It's a completely different and unrelated set of fraud.

Trump could be criminally charged, in separate cases, for both of the fraud schemes without double jeopardy attaching.

With all that said, Trump can choose to testify in this case. Doing so waives his 5th Amendment rights and subjects his testimony to challenge of his honesty. That allows the admission of evidence relating to his honesty (via cross examination), as my article described, that would otherwise not be admissible.