If you are flexible on when and where you eat (and are ok with counter service if something doesn't pan out), then you can most definitely be flexible. This is our preferred method of touring as we tend to be more spontaneous and not very structured. We do make some ADR's for must do restaurants (like California Grill at fireworks time) but otherwise we go where our bellies and open reservations lead us.
For our trip in August, we currently have exactly 2 reservations made. Day of reservations are much easier to come by then people imagine. Rather than just walking up, use the app to find open reservations an hour or two before you think you want to eat. I'd suggest playing around on the website to see what is available on any given day to give you an idea of what could be open. Below are some examples that we get last minute regularly.
MK - Liberty Tree, Skippers Canteen (we've even gotten BOG lunch a few times)
MGM - 50's or Hollywood Brown Derby
Epcot - Via Napoli, Tokyo Dining, Rose & Crown, San Angel
AK - Tiffins, Yak N Yeti (the Landry's card also works great here), Tusker House
Resorts - Jiko, Artist Point, Whispering Canyon, Narcoossee's, Kona, Grand Floridian Cafe, Flying Fish
Springs - Ragland Road, Morimotos, Boathouse (note for Ragland and Boathouse you can also make Open Table reservations and there is no penalty if you cancel day of)
Specific to the ones you mentioned, we've done day of at Tusker though SciFi can be a little harder to get but we have gotten day of availability (start checking the night before if you don't have an ADR). We haven't done Garden Grill, but I see availability often.