You seem to be in the habit of blaming everyone but yourself. For months you’ve been ignoring everyone telling you to just stop punching yourself in the face, and now here you are having to live with the consequences of doing so.
He was paying maintenance during the time that you were primary carer, and in fact he was still voluntarily paying you maintenance when it switched to 50/50. You’ve already had more than what was owed to you.
The past is irrelevant, as is what you personally think to be fair. You haven’t been the primary carer for a while now, whether you choose to accept that or not. I’m also not sure why you’re so convinced that you did more admin, given that he was covering their private medical insurance and doing everything that came up during his time.
He’s got a very good chance of getting child benefit for DC2, and so on top of having to reimburse him for maintenance he overpaid, you’ll also have to pay maintenance. Start saving, I guess.