Hello! There are lots of great breakfast options. The one that works best for you will have a lot to do with your touring style.
When I'm with my family, we like to make sure everyone has eaten something before we head out to the parks, and we usually try to get to the parks as soon as they open. For that scenario, doing a Garden Grocer order for easy food to eat in the room or using the resort food court is nice and easy.
However, when I'm by myself or not with kids (like you will be) I prefer to just head to the parks and eat more interesting things there. For example, I'll get a fruit & Nutella waffle from Sleepy Hollow in MK after I've done a few attractions. I like Sunshine Seasons for a quick breakfast in Epcot. Each park has a Starbucks location inside, and some of them have a Joffrey's coffee cart outside.
If you're wanting a real, sit-down breakfast then there are lots of great options. I love a good breakfast. It's a trade-off though, because unless you book a really early breakfast, you'll miss low-crowd touring time while you eat. It's a perfectly valid choice though, and we've done it many times! My favorites are Crystal Palace and Be Our Guest in MK and 'Ohana at the Poly. I've also heard great things about Garden Grill in Epcot, the Wave at the Contemporary and Trattoria al Forno at the Boardwalk.