OP, I think you are getting quite worked up about something which, in principle, you are completely correct about, but in practical terms, is a very small thing. To me, what you're telling us here isn't that you feel annoyed with your husband for making a very small financial gain at your expense, but because of his attitude towards money more generally.
Two things are jumping out at me here.
The first is that whilst he is happy to use your money to benefit the family, e.g. by having a joint savings account linked to your individual account to get a better savings rate, he sees his money as his money alone, which has nothing to do with you. Opening a savings account linked to your joint account for the benefit of himself and his siblings without even telling you is secretive. At best, it suggests that he doesn't think his money is any of your business, and at worst, it suggests that he actively wants to keep you in the dark about his personal finances.
The second thing is that your joint expenditure comes out of the joint account, to which you both contribute equally. I'm going to go out on a limb here and say I bet your husband earns more than you - possibly substantially more than you. Not just because you're a woman, but because his attitude over this suggests to me that if you were earning more than him, he'd want you to be contributing more than 50% to the joint account, but if you're earning less than him, he's happy for it to be 50/50.
If you earn less than him, he has more disposable income than you because he is putting a lower proportion of his salary into the joint account each month, leaving him with more money for savings, investments and fun stuff just for him. It also means that, should you ever divorce, he's probably sitting on a much bigger pile of personal cash than you are, with which to pay a shit hot lawyer to get him the best possible financial settlement.
If you earn less than him, why is that? Do you or have you ever worked part time to look after children? Have you ever been a stay at home parent? Have you ever had a period where you earned less than usual or nothing at all due to being on maternity leave? If so, even if he made more of a financial contribution during that time, I bet he wasn't paying your pension contributions, was he?
If you earn less than him, how do household chores get divided? Does he do an equal share of childcare? Cooking? Cleaning? Laundry? How do you divide these things up? Because if you do more of the "wife work" on the grounds that he earns more money, but he's still expecting you to contribute 50% towards your shared expenses, he is profiting from your unpaid labour.
Maybe I'm projecting here, but his attitude towards the savings accounts plus the fact that you each contribute 50% towards the joint account when it is extremely unlikely that you both earn exactly the same salary suggest to me that some of these other things are also likely to be true.
And if that's the case, you have a bigger problem than who is getting the interest on this savings account.