How is that going to help retain all the new hires at the two new Gigafactories and provide them with an economic reason to stick around a number of years and make a career of it? He wants the factories to hit the ground running and keep running long into the future. Optimus will not be ready in time to solve labor challenges at these huge new factories in the medium term.
Elon is a "big picture" kind of guy, and he likes to accomplish multiple things with a single stroke and move onto bigger things. He knows any impact to share price will be transitory in nature. He's not the kind of guy to tiptoe around while he sells his shares in dark, secret shadows to get every last dollar out of the transaction for himself and screw over his loyal employees. Open public markets exist for a reason and the normal ebb and flow of the price is as natural as the supply and demand of the shares.
Be aware that when selling billions of dollars worth using dark pools it is necessary to reduce the price substantially in order to move that much stock but that is not reflected in the "official" share price. I think Elon is a straight-forward guy and figures the open public markets exist for a reason, they should function as intended. It's actually a subversion of open markets to do large transactions off the books and away from the very markets that you and I have to use.
One additional note: I would not be surprised to see one last "hurrah" attempt by those who are disappointed Elon's selling has ended to keep the share price down. They might figure this will send a powerful message that it's the actual company that is weak, not just a temporary downdraft caused by Elon needing to raise billions in cash and de-risk his Twitter purchase going forward. Any weak hands still holding a portion of their stake could become unnerved and give up their shares.
This could be difficult to pull off if there is unprecedented buying pressure but don't for a minute think they wouldn't try as it really would send a powerful, even if false, message. They don't want this downdraft to be any more temporary than absolutely necessary! Stay strong!