No, doing it yourself does not void your warranty. But document carefully what you did and when. I'm pretty thorough: I keep all receipts for oil, filters, etc.; I note the mileage and date I did the work; and I take a dated digital picture of the front end of the car on ramps with the drain pan sitting beneath it and four bottles of motor oil sitting next to it. Maybe that last step is a bit anal (and a sign of my own paranoia), but it's simple and should be very convincing if my DIY maintenance is ever questioned.
Presumably the $70 quote includes at least the Toyota recommended 5K service (I would hope). That service interval includes tire rotation and visual inspection of brakes. I skipped both of those at 5K. I rotate my own tires every 10K, unless I'm too busy, in which case I'll have the local tire shop do it. I've done a 10K tire rotation on every front wheel drive car I've owned, with excellent, even tire wear. I eyeball the disk pads when I rotate the tires, and I figure the annual state safety inspection is otherwise sufficient for brakes. The Prius is inherently easy on brakes (see below), made even more so in my case by my driving techniques.
What you're experiencing with the brakes may be the Prius' normal transition from regenerative braking to friction braking. Above 7-8
MPH with light braking, engagement of brake pads and shoes is limited. Instead the otherwise wasted kinetic energy is recaptured by the hybrid system to help charge the battery. As you brake slowly past the this threshold, you will notice a slight "grab" as friction braking takes over. I'm not sure there's any adjustment for that. (Maybe others here can confirm or refute.)