I don’t think there is enough context or information to take a properly -informed view, but this does seem like a bit of a storm in a teacup, to be honest.
It’s 15k, in fact only 7.5k as he’s agreed to save half of it jointly (in a fictional “joint ISA” but let’s just assume that there is another, true joint savings vehicle where it can go).
OP describes them as “comfortable” and the DH does work, he just isn’t the main earner. The assumption is that his income is also treated as joint family money.
We don’t really know what a “huge” mortgage means, but it’s unlikely to be out of kilter with their joint earning capacity pre-inheritance. If they have over-stretched themselves with other debts that was OP’s fault as much as her DH’s. We don’t know the proportions in which they contributed to the deposit for the family home.
We don’t know whether the DH does less, exactly or more than a 50% share of the childcare and child-related physical and mental load.
We don’t know if the DH is insisting that all OP’s earnings go in the joint pot or whether he’d be perfectly fine for her to spend 7.5k on something entirely for herself.
I doubt very much that 7.5k spent on the mortgage would make much of a difference to their daily and longer-term financial position in the grand scheme of things. It might be better to use it to do some work on the house, but the other half could go towards that.
OP maybe instead of a theoretical calculation you could sit down together and agree (eg) we really need the bathroom re-done, how about you price up, organise and pay for that and then spend/invest whatever is left however you like?
The desire to invest in Bitcoin is a bit naive, but unless he’s a total idiot surely he’ll do his own research first (and hopefully decide against it). It still has a better potential return than buying a luxury item or going on holiday.