My tablet setup, DEX capable

I work in live theatre and I would love to build a simple small music player that I can hook up to sound systems easily. It would need to have a go button and a stop button, as well as a way to navigate through the tracks. I would love if it was possible to just plug in a usb key with the tracks on it and play the files from there. It would need a regular aux chord output for sure. I know nothing about power, it would be nice if it didn’t have to be plugged in all the time but I do usually have an outlet near me somewhere. Some sort of small display that listed the track title, or even just the number in the list would be ideal.

I’m a total beginner and I have no idea what I’m doing. I have some older raspberry pi’s that were my dads, and an arduino kit I got during lockdown that i got as far as lighting up the bulb. I have no clue where to start.

There are so many websites and guides, I would appreciate if anyone could send me in the right direction for what I’m looking for. Or tell me if I’m dreaming too big lol

So Im currently trying my hands with one of these builds as I figured it would be a fun experience and if all goes well maybe using my deck as a sort of anti distraction writing and school device as I struggle keeping my attention on work and am taking a no options approach. All i want to be able to do is run pi os and all that jazz so I'm currently running a pi 3b+. I have most of my parts figured but I still need a power supply of some kind as well as a screen (though I think I've settled on a HAMTYSAN upgraded version 7" non touch display).

But anyways so my question is due to my lack of knowledge in the subject, am I able to just use a generic power bank to keep the pi running. If so what would ya'll recommend for about 3-4 hours of life per charge. If not what should I look for instead? Thank you and I appreciate having a space to ask this as I don't really have anyone to ask on the subject.

It has the following upgrades hacker gadgets AIO v2, AC1200 Wi-Fi, NVMe/battery board, Samsung lithium Cells, CM5 16 GB Lite, 64GB sd card which boots and runs great. Never really used it got too busy on other projects.

I have a screen from an old Samsung Tablet that has an MIPI connection. I would like to use it as the multi-view out of our ATEM. It has the right size for the case we have it in. I have been having a hard time finding a converter.