So sorry you are going through this OP.
One really strong piece of advice: given how incredibly reluctant he has been to show you the statements, I would not accept a pile of printed statements as sufficient proof of where the money has gone.
Documents can be edited, transactions can be removed, screenshots can be selective etc. If transparency is now a condition of you even considering continuing the marriage, I think you need to actually sit next to him while he logs into the relevant banking/credit card accounts and look through the transaction histories together. Not statements he has downloaded and prepared beforehand.
And I think there is a much bigger question here which I’m not sure you are fully allowing yourself to ask yet - where has £25,000 actually gone?
You’ve said that towards the end of the month you were sometimes having to put food shopping on your overdraft because there wasn’t enough left in the joint account. You paid for your family holiday from your own savings and he is repaying you £30 a month. Your children are wearing hand-me-downs and inexpensive clothes.
There is absolutely nothing wrong with any of those things, but they paint a picture of a family living relatively modestly and watching the pennies. Meanwhile, unbeknownst to you, £25,000 has apparently been spent somewhere else.
If his explanation is that this all arose during one year of maternity leave, £25,000 divided over 12 months is about £2,083 every single month. That is an enormous amount of additional spending, every month, on top of your normal household expenditure.
If £2,000 a month were being spent on clothes, meals, things for the house, the children, family activities etc, surely you would have seen the benefit of at least some of it. There would be things to point to.
So before accepting “maternity leave” as an explanation, I would want to understand exactly what that means. Was the family genuinely short by £2,000+ every month? What was your household income before/during maternity leave? What bills increased? What exactly did the credit cards pay for?
Because if you cannot see where £25k has gone, then unfortunately you do have to consider spending which wouldn’t necessarily be visible within the household — gambling, subscriptions, online spending, another debt being serviced, cash withdrawals, adult services, or something else entirely. I’m not saying it is any of those things, but his refusal to let you see the transactions means you currently cannot rule anything out.
And transferring it to 0% is sensible financially, but it only deals with the interest. It doesn’t answer the much more important question of how £25,000 was accumulated without you knowing.
Personally I wouldn’t even be making decisions about forgiveness yet. I’d be making decisions about facts. Full disclosure of every account, every card and the complete transaction history first. Then you can work out what exactly you are being asked to forgive.