it isn’t defamation if the defendant genuinely believes it to be true
No. Truth is an absolute defense to defamation, but this means that the defendant must prove to a jury that what they said is actually true, not merely that they believed it was true.
If the plaintiffs can prove that what the defendant said is false, then the defendant can defend themselves by showing that they were supported at the time by a reasonable body of objective research/investigation.
So for instance if you publish a story about “My neighbor, the murderer”, you could cite the court case that found them guilty. Not just police accusations or your genuine belief that they committed murder. That’s why reporters usually say “X allegedly committed a crime” before they are tried.
Note that a different standard is used when plaintiffs are celebrities, but that won’t apply in this case.