In the interest of battery life and redundancy, I think it might make sense to have 3 devices.

Ereader with an e ink display for reading, a lot of these can last days or weeks on a charge easily

An mp3 player for music. I don’t know what the current state of mp3 players is, I suspect a lot of no-name imported garbage, but over a decade ago I know my iPod used to go days or weeks on a charge with pretty heavy usage. Probably look for whatever has the least bells and whistles you can find- no touch screen, physical controls, etc. if you’re up for a bit of tinkering I’m pretty sure there’s a pretty active scene for people modding old iPods with better batteries, more storage, etc. that would probably be a great option.

A tablet or smartphone for movies, or possibly a laptop (I’m not an apple guy, but I’ve heard MacBooks have pretty insane battery life these days.) Keep all the wifi/cellular/Bluetooth/gps, etc. turned off, keep it on power save mode, disable anything you can that you don’t need to watch movies. Unfortunately if such a thing as a dedicated video-only tablet exists, I couldn’t find it with a quick search. If such a thing can be found, I’d probably recommend that.

A dedicated device that does one job well will usually be more efficient at that thing than a multipurpose device like a tablet, smartphone, or computer that needs to be able to do it all. An mp3 player only needs to be able to play music, it doesn’t need to be running a full-on OS that’s capable of sending emails, making phone calls, playing games, etc.

Also that way if one of those things does die on you, you still have the other 2.

I saw in one of your other comments your concern about a tablet having a bigger screen would be a bigger drain on battery life. That’s true to an extent, bigger screens draw more power, but since the whole device is bigger they can compensate with a bigger battery. I haven’t exactly done an exhaustive survey of tablet battery life and don’t care to look into it, but in my (fairly limited) experience, they usually pretty much at least break even or surpass phones in battery life. I have a cheap tablet that I really only use for reading it lives in my bag, usually in my car, often forgotten about for days or occasionally weeks at a time, and doesn’t exactly get heavy usage, but it usually can go at least a few days without a change, even with WiFi and Bluetooth left on. If I’m not using it at all, it can sometimes go a couple weeks just sitting idle. It’s usually good for at least a couple hours of streaming HD video, with WiFi turned off and 720p video on internal storage I imagine it’s good for at least a couple movies.

WiFi and cellular data are pretty big power drains too. I know when I check my battery usage on my phone that probably accounts for about ⅓ or so of it. Having those turned off can go a long way. Jailbreaking/rooting your phone to disable unnecessary services probably wouldn’t hurt, but that’s probably a drop in the bucket compared to just keeping your device offline.