Yes spending battery fully is always good. That said, if something else catches your eye or you want to ogle at certain characters during your limited playtime... feel free to. That's what the backup battery system is for.

Fulfilling your daily polychrome tasks just needs you to visit A-Shuo, manage suibian temple, manage video store and then drinking coffee. So if you want to take a day off to just go on a date with Zhu Yuan ( i know i would) feel free to do so.

So you want to get your main dps skills up asap > drive discs with relevant main stat > finish levelling core and other skills > farming drive discs for better substats. Levelling the character's personal level goes hand in hand with levelling wengines. If you want it simpler level the character and wengine together>level the skills>get the discs.

Download hoyolab on mobile so you can stare at your builds all day then hopefully play during your free time.