Other than your post in the ground the other key item to help stiffen it up is your rail cap.
You have it breaking on that middle post.
Is that deck under 20 feet? If so one piece that cap. If not at least break it on a post as far away from the center as you can.
Can’t tell with big black dot in the way but if that ?x4 is breaking on that post it should go too.
Getting rid of the 1” cap and go with 2x6 or even better 2x8 ( more meat left after notching for post) would go a long way to stiffening it up. 2x8 will also give you more room to spill drinks.
Do you really need the dbl. 2x10 on top? Make it 1x and lose 20lbs up there, that would help.
Notched post wasn’t the best choice but all this stuff I mentioned "I know" will help.
Getting rid of the notched post and doing the other things I mentioned is as about as good as you’re going to get it, other than going to 6x.
A Picture Is Worth A Thousand Words
Especially In The DIY Chatroom
Please include your basic location in your profile. Some of the answers given are specific to a Region or State.