Sorry to intrude in your forum, but your right, it is chickweed from the looks of it. I dealt with it alot when I lived in SEPA. You have to use a product with a specific active ingredient to take care of it--triclopyr. Ortho has this product that contains it that I linked below. The general Weed B Gon products that you use for dandelions, etc will not take this out, they may stunt them, but not kill them. Also regarding your granular pre-emegent--- chickweed germinates in late summer/fall and then sits dormant all winter and then explodes in spring--so the pre-emergent wont have any effect on chickweed. It will be good for all the spring germinating weeds though. You would need to do late summer pre-emergent also if you wanted to target chickweed (and other fall germinating weeds). Hope that helps a bit.
https://www.homedepot.com/p/Ortho-Weed-B-Gon-16-oz-Chickweed-Clover-Oxalis-Killer-for-Lawns-Concentrate-0396410/203686814