@resengsc basically you are already on the latest international firmware.
What you could try to do, is to reinstall the whole firmware with the same procedure as the upgrade one (https://meizufans.eu/f-a-q <- Pictured tutorial).
Also try to calibrate the battery sensor, by fully discharging the phone until it turns itself off and then charging it while the phone is still off.
I was thinking to reset the phone but I already installed many apps and organized the phone quite a bit. I don’t want to lose that. Is there any way to keep all the installed programs but just upgrading OS? I heard that FlymeOS 5 also helps with the battery. Do you suggest to install that version (it is still beta?) or just stick to the current stable version?