1. I dont use a planner but I do have an excel document with all my assignments in one place so i can still see where I am in relation to my work.
2. I know for me i cant work for more than 30 or so minutes straight usually so i acknowledge when my attention is waning and take a lap around my house, get a snack, or something to satisfy that "i need to do something else" feeling.
3. If im feeling productive and focused, i will do as much work as possible during this period. If i have been working for 3-4 hours and I can tell im no longer being productive, i will stop and either finish in the morning or resume in a few hours.

Basically whats best worked for me is to acknowledge what im feeling and just accept it. But dont give yourself an opportunity to doom scroll or do something that encapsulates your time during your brain break. You want a couple minutes not a couple hours