> And (again, just a personal advice) you should only accept an offer if you plan to immediately forward exercise all the offered options
horrible advice
Why's that? (Not trying to be snarky, generally want to know more)
You have to pay for the privilege of owning the stock (exercise price) plus whatever gains the valuation has increased it to. So, if you got options at $1/share and bought them when they were privately valued at $10/share... You have to pay $1 per share plus $9/share in whatever your marginal tax rate is. The upside here is that if the company goes public later at $20/share... You only pay $10/share in capital gains tax rates than $19/share in marginal income tax rate. (Assuming you hold them for a year after purchase)
Downside? You just paid tens of thousands of dollars in hard cash and taxes for lottery tickets. The company could still go under, you'll never sell unless they get acquired or ipo, and you'll be out all that money you spent.
In general, I think it's horrendous advice to forward exercise. Great way to lose money.
If your exercise window is only 90 days, then you need to exercise at least some of your options and file an 83b. Otherwise you'll be stuck at the company until a liquidity event, which can take 10+ years. If the company is successful and you need to leave early for any reason, you're potentially throwing away millions of dollars.