Quick service (a/k/a counter service) restaurants can get quite busy from 12pm-2pm, and 5pm-7pm, when the lunch and dinner rushes are going on. Arriving just a few minutes before or after the rush can make the difference between no wait and a 20 minute wait, but they do their best to move people through quickly.
You can't make advance dining reservations (a/k/a ADRs) for QS meals (except for Be Our Guest breakfast/lunch in the Magic Kingdom, which gets booked up months ahead), but many of the in-park QS locations have "Mobile Ordering," which allows you to log into your My Disney Experience account app and place your dining order ahead of time, while you're on your way to the restaurant. When you arrive, you tap a button in the app that lets the restaurant know you're there, at which point, your food starts to be made immediately, without you having to wait in a line to order. You can "pay" via the app, with DDP credits or with a credit/debit card. The list of eligible restaurants can be found here:
https://disneyworld.disney.go.com/guest-services/mobile-food-orders/
I suggest clicking on a few of the restaurant names so you can look at the menus and get an idea of what sorts of items you can expect at QS locations. (Most will offer a variety of things, with small differences based on location, size and theme: you'll typically find at least one burger, a sandwich or two, a couple of more healthful options like salads and vegetarian dishes, plus sides, drinks and desserts). Because my family only visits WDW every 2-3 years or so and the menus change frequently, I jot down a list of QS locations and notes on their menus (e.g., "flatbreads and salads," "seafood" or "tacos and fajitas") on the back of my written touring itinerary for each park, so my family can make choices about where to eat on the fly. I even put an asterisk next to those with indoor seating, so we know where to go if it's super hot or rainy. There's plenty of variety as long as you know where to find it.