Battery will vary by individual and what you're doing on the tablet.
I personally don't consider a wireless charger worth it, my stock charger works just fine and I don't mind the extra second it takes to plug it in. Plus I don't have anywhere to leave a wireless charger, so if I have to move it all the time seems pointless in my case.
I prefer to use a sleeve. If I'm watching a movie I use a cheap case from eBay that doubles as a stand.
Because you want to. I use stock ROM these days. People flash custom ROMs for various reasons, only you would know why YOU should flash.
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