Typically exercising options gets taxed as regular income on the difference between option strike price & fair market value.
If OP had something like $300k of options, that'd be about a $100k tax bill assuming the strike price was appropriately low. Not a very big payout on a 9-figure exit, but also not pocket change for the taxes.
Well, if the "9 figure" was 100M, then 300k seems fair, since after 10 years of fundraisings and even minor dilutions, it's very easy and expected to go from 1% to 0.3%.
On the other hand, if the amount was 500M, 300k seems very low, hinting to a final ownership of ~0.06%. If that was true, even after considering all dilutions, that makes me think that OP didn't negotiate a fair equity package to begin with, since as employee #1 you should definitely have at least 1%.
If OP had something like $300k of options, that'd be about a $100k tax bill assuming the strike price was appropriately low. Not a very big payout on a 9-figure exit, but also not pocket change for the taxes.