If you go into WhatsApp and tap on the 3 dots on the top right Go into settings Then chats Down the bottom is chat backup. Just tap on this. At the bottom You can also select to backup videos as well May take a while if its not been backed up for a long time.
Installing the app from fresh, it will give you the option to restore, which will be from your backup.
Honestly, yes it is good for the s21 battery capacity. 6 hours and 6 mins on screen time and 9 hours 44 mins off screen time I would be happy with. Considering Instagram and tik tok are quite heavy on battery.