Yep. That's the conundrum. If the company is doing so obviously well that the shares are likely to be worth something in the future, then they are almost certainly already valued high and have a huge potential tax hit. In hindsight, most hugely successful starts were still a big gamble in their early days when the share price was low and it wasn't the smartest thing to throw thousands, tens of thousands, or even more of your own cash at them.