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.
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.