It seems to me that the correct remedy in this case would be to have a representative of the court preserve specific data requested by the offender, as long as it isn't criminal or harmful, at the offender's expense.
I don't think wiping an offender's phone is justified in most cases even if it might hinder them in potential subsequent crimes, but it does seem justified in this case where the phone contains significant material harmful to the victim of a serious crime.
I would go so far as to preserve the complete, forensically-vouched data image that the original prosecution relied on. The good, the bad, and the ugly. At the offender’s own expense, as you mention; for as long as they want to keep paying before they bring their lawsuit (or not).
If the claim is that some of the material there evidences police abuses of some sort, you can’t really let the offender pick the stuff they feel supports their claim and wipe the stuff that might weaken it.
And if there’s criminal material mixed in with the other, non-offending data that the offender claims they need, it seems like you kind of need some kind of adjudication process on an item-by-item basis.
They’ve clearly worked out a way to manage custody, access, and review for the purposes of the initial prosecution. Let the offender fund it under the same terms until they either fish or cut bait. Maybe if their claim of police abuse works out, this might be among the costs the court could award.