What personal data are opposing solicitors allowed to retain?

I was a litigant in person during my divorce proceedings and the financial settlement which are now concluded. My former partner's solicitors requested a lot of evidence (over an above what is required with the form E) from me during the financial proceedings which, having nothing to hide, I provided. With the benefit of hindsight, I think their requests were excessive as much of the information was never used in the proceedings. However, as a result, they now have a wide range of personal data including sensitive information like credit card numbers, account numbers, NHS number etc. Now the proceedings are over, I have requested that they delete any documents (eg credit card statements, doctor's certificates etc) that are no longer required, but they claim they need to keep every single document for their records (including old versions of ones that were updated in subsequent disclosures).

I recognise that solicitors need to keep sufficient records for SRA audit purposes etc. but this seems excessive to me and does not balance their risks with mine eg. if there was a data breach. In addition, presumably it would be a data breach to release these documents to my former husband now we are divorced (regardless of whether he previously received copies)?

I complained to the Information Commissioner's Office who asked the solicitors to do more to meet my request but they have dug in (refusing to even redact sensitive identifiers that they could have no possible use for) and the ICO has now said there is nothing more they can do, which is disappointing.

Any advice welcome.