Brave has a builtin ad blocker, though not as good as uBO in Firefox. You also have Cromite, which uses Adblock Plus and also the Kiwi Browser, which supports extensions so you can install uBO.
The battery is probably about to die or is in a bad state. I doubt it has something to do with software. Google also won't void your warranty for using a custom ROM. UNLESS you somehow brick your phone or cause permanent damage with root.
Agreed.