For the various major life stages (big ticket items), whats the expense range I should be looking at when planning for it?

Instead of planning for future expense and working backwards, I suggest focusing on doing a bottom up approach.

Focus on ability to make money and save.

When you're single, if you can't save 60% of your income, you can't afford to get married.

When you're married and can't save 40% of your income, you can't afford to have children.

When you're married and have children, if you can't save 20% of your income, you can't afford to retire.

This works better psychologically, future prices are honestly a guesstimate, for all we know uni education fees could be free 20 years from now.

So save that % first, and then when you're there, you know your affordability.

It seems like most of the big ticket items you have identified are related to starting a family. Aside from that, you might want to think about the down-side protections ie. illness etc, if not already covered with insurance.

Now with that said, I agree with Hariz that it really boils down to how much you can afford.

Take weddings for example. It can be super simple and lowkey, or super extravagant. The budget range would then be anything from thousands to a six-digit figure.