There's 1 year cliff for vesting but apparently one can excercise at the current strike price on day one and that's pretty common among other startups with folks who want to avoid the AMT

It's "83(b) election." It's interesting a late stage startup is offering this. But do NOT do it.

Do this only for very early stage startups where your total expense might be a few thousand dollars for maybe 1% or more of the startup. At most, you lose a few thousand dollars.

Spending $100K to optimize possible future taxes is a bad idea. At this stage, you should just think that future AMT is a good problem to have. Put the $100K in safer instruments.

At some point in the future, if it becomes clear that the company is on path for an ipo I could pay for it then.

I will hold off until then.

Imo you are taking on a huge risk. Your shares will be worth nothing the majority of the time. In the future if your shares are worth a lot and you need to pay taxes that will be a nice problem to have.

That's a common way to save on taxes by paying a bit upfront. It's usually not done at such a late stage because of the cost when the strike price is high. It's more common in pre-A or at least pre-B companies when the strike price is smaller.

I certainly wouldn't drop 100k unless they offer a commensurate signups bonus (the 100k goes to the company so they could give you a bonus of $100k if you exercise the options which should help, though you'd still be on the hook for the taxes on that bonus).