If a company hasn't IPO'ed, an 83(b) election may also make sense.
After whatever lockup you have some fraction of shares remaining. When you sell those and if the price is greater than the IPO price then you have capital gains on whatever shares you still hold.
You still owe tax from the IPO. My point is the company will withhold shares from you, but you might need to pay more if they underwitheld for the IPO. This has nothing to do with your capital gains from selling your remaining shares.
All said, the better companies offer partial recourse loans to early exercise.
This is days later, so not sure if you'll see this, but could you explain what QSBS has to do with it?