Which is better for Pre-IPO shares: Carta, Forge, Equity Zen or buying direct from employees

Has anyone used any of these companies to buy Pre-IPO shares? Or is it better to directly solicit an employee at a Pre-IPO unicorn company. I missed out on a few opportunities this year and want to make sure I do not miss out in the future.

How does one evaluate fair share price Pre-IPO. Is there a calculator for this?