I can tell you what happened to me. The first days the computer keep itself updating, that takes a lot from the laptop.
Check the task manager for cpu usage, the first days there is a lot of taks running under the system. There is also the last windows update, it takes a lot. Then it should calm down.
The screen brightness doesnt take a lot from the battery, almost all goes for the cpu, if it is running at 90% the battery drains in 3 hours. See the post made by chupa about it, it helps a lot.
And when streaming use edge, it saves more battery life.