Could pre-emergent that I spread march 22nd have killed the crabgrass already??

[Lawn-N-Order]

So I’m finally getting serious with renovating our lawn. We moved in a few years back and were left with a 1/4 Bermuda 1/4 st. Augustine 1/2 crabgrass and mix or weeds ect. So I laid the pre-emergent down and now I have a bunch of what I think to be the crabgrass. Is it possible it worked already? I would think so. So now that about 35% of my lawn is dead I was thinking what the next step might be? Should I detach or I guess basically rip all the dead stuff out and have bare soil and add seed or get a pallet of grass? I want all Saint Augustine by the way, I’m going to try to upload some photos as well. Thanks for reading by the way I am very new to all this, and very excited to turn the yard into a gem!