Feedback for rain garden plan

St Louis. Zone 7.

I have this area of my backyard that will get standing water for about 24 hours after a heavy rain. The main area is at the bottom of gentle slope. 50' long 12' wide but sort of an oval shape.

I like the idea of a shrub or two in the middle sort of the anchor everything. I know I want button bush, but now sure if I should do two or find another good shrub to fit. I arranged by height to have taller things in the back and shorter in the front with some variation for interest. Also intermixed for color and bloom season. (Or tried to at least). I have a small black gum and then a few bald cypress trees I just planted at the top corners.

Looking for feedback on another shrub to include? Other recommendations for plants or alternate plant locations? Also what are some good grasses/rushes/sedges I could interplant and do I just fill in empty space or is there a better strategy?

Sorry for bad picture of design. Also including picture of the area of the yard from our patio. The area im hoping to plant is where you see the water between the oak and the juniper.