Some dining opens at 5:30 am EST (so 4:30 am CST). Space220 opened at 6 for our March trip, but last Nov it opened at 5:30 EST. I booked an after dessert party at 5:30 am for our upcoming trip. Other parties didn't open till 6 am. Other people reported treats and seats opening at midnight (Up Close with Rhinos as well).
Don't filter for anything (that feature won't work when the system gets overloaded), but you can try. Open each restaurant screen on a separate tab and have the app open (and preferably another device). Also, if you don't see your dates don't panic. Go to myplans page so the app/website checks that you have tickets and then go to the restaurant screen and click 'check availability'. I never saw my dates highlighted until I clicked the check availability tab and scrolled through the calendar. As a backup, if you're on a desktop you can also type in a date when you check for multiple restaurants on the orlando area restaurants screen. This screen will most likely freeze on one device when a lot of people log in, so have backup plans. Credit card should be on your profile already set to go. If you get stuck beyond the select party screen, don't loose hope, click the back button (up to the select party screen) and it holds the reservation for you for a few minutes.
Space220 will most likely only be available 60+6 days out, so book that first. Ohana isn't as in demand given recent bad reviews, but I would book it second. Story book dining and Topolinos breakfast have been challenging. Flying fish and steakhouse only showed 4 seaters. Space220 lounge showed 2 seaters even when they had multiple available (I couldn't book for 4 at 60+7)