It sounds like as a couple, you haven’t made the decision to marry and/or have children.
Therefore, there is an argument for 50:50 - but that doesn’t mean you have to agree it’s right for you.
But… as others have said, you’re not paying 50%.
As you’ve been together 10 years, his kids must be well into the “not cheap to feed” age.
Don’t kid yourself that he’s oblivious.
Sure, you might not stop to think just how much kids add, or your supplements - easy to realise they’re ££ but think, “oh probably balanced with her having fancier shampoos.” I think someone could be genuinely surprised at the different. But no way can you earn 5x your partner and be oblivious that they’re paying the same and earning less. More likely he thinks like me: 50/50 is one valid model. (though again: it’s not 50:50)
I would approach it without a suggested split. I’d simply go in with - my finances are too tight, I have paid 50/50 for your kids for years, and now that you’re earning far more than me it (a) is unfair and (b) is unaffordable, so I need to review how we split things with you.
If he kicks off, or does anything other than say yes, really - then he’s using you. No need to waste your time working out a fair split - he’s not worth it.
If he says you’re right… then you can start working out what’s fair.
I earn 4x my husband and have more outgoings (my children, not his) and I pay far more than him - so my money is where my mouth is!