Set some confidence targets with hard times before just raid by early october. Ask your guys who plans to be level 60 by 15september and set pre raid BIS dungeon farming groups up so some guys have a set target.

Set up some lower level guild dungeon runs with those guys so they can start get a feeling on how eachother plays, have a good time and some of your more casual guildies will see the fun in guildchat and may be inclined to join in on the fun.

Run your guild how you described it and why people joined. If you said it's casual don't be too strict and accept that some of your better players are gonna leave. If you said you want to do progression you've gotta enforce it "mains lvl x by y date" or youre gonna lose your key guys that make progression possible. Help out the guys who suck but are trying (not everyone knows what wowhead is or rotations or specs).