Wow, he's done every single thing to benefit and safeguard only him financially rather than you both as a couple. You, the mother of his kids and their main carer, who has given up working full time to provide childcare for his children... he'd happily see you be financially fucked if you split or he passed away.
The only reason I would marry him now is to safeguard yourself financially but that feels incredibly hollow and sad for you. So I would be pushing for signing legal documents (next of kin, a will etc) jointly that protect you in the instance of a split or a death.
He will say (again) that he won't do it as he's 'going to propose' but he won't do it. Then he'll start to blame you saying he's not sure he wants to marry you as you've pressured him etc. Textbook.
I'm afraid I'm not sure what the answer is really as he's very clearly not willing to protect or safeguard you financially despite you making more sacrifices than him due to going part time.
You need to go back to full time and tell him that childcare costs are a joint responsibility. There's absolutely no reason they need to be covered by your wage rather than his, as if your career / pension / future is worth nothing if he is any worse off during the few years you'll need childcare.
As I say, I wouldn't want to marry such a selfish prick and I don't think he'll budge so I don't know what the answer is.
You'll need to reassess your prejudice against single mums and start seriously planning how to be one. It's doable, otherwise there wouldn't be any single mums in full time work - but there are plenty. It's tough and more difficult than being in a couple with dual income but that's a consequence of trusting someone else totally without safeguarding yourself.
I would be demanding you both sort a will, life insurance, property rights etc. But that demand won't be met with him acting on it as he knows there's no benefit directly to him in doing so and he is a cunt.