There's a simple solution here: make employee options liquid during major fundraising events. So when you close your round B/C/etc, give employees 30 days or so

There's a simple solution here: make employee options liquid during major fundraising events. So when you close your round B/C/etc, give employees 30 days or so to exercise their options at the pre-money valuation. If you're nervous about employees exiting too early, pick a high valuation (say, $50M) and the option exercise clause doesn't kick in until then (at that point dilution shouldn't be an issue).