You’re right that it’s a marital asset if you divorce.
But until then, it’s his money. Gift, inheritance - call it what you like, it’s his money.
Honestly, if I inherit some money, or win on a scratch card, or (more likely!) get a work bonus, that money is mine - not ours. I’m potentially being made redundant next year, 5 figure sum will be mine, in my account. It’s probably relevant that this is my second marriage with no joint children.
If I inherited that level of money tomorrow, it would go into my pension. (bar £10K spent on a fab holiday!!) If my husband inherited it, I expect he would do the same re holiday, but then split it 3 ways as house deposits for his 3 boys.
We would talk to the other person about our plans, and listen to them, but bottom line - we would both see it as our own money, and expect to have the final veto.
What is different for us than you, is that we’ve always operated like this, and we have similar attitudes to money. I might choose pension whereas he’d support his boys... but neither of us would be investing into a flaky looking start up when we’d backed 2 failures before! And we also don’t have your history where your* money (bonus, redundancy) has been treated as joint.
So although I think it’s right that it is his money - I would seriously question whether we had an outlook on life and relationships that was compatible with staying married.
*I’m assuming that he’s not an inexperienced high risk high reward investor who can afford that profile. I’m not against losing on 2 start ups and then winning big on a 3rd! I’m more concerned with whether you can afford that strategy. I’d say that as he’s relying on your pension (cheeky fucker!) that he cannot afford that strategy