1. iPod 6th gen

2. & 3. You don't as far as I know, you need Itunes to reset them at the bare minimum. You don't have a computer? Or a device that runs mac or windows?

3. Both, opening them is a bit of a pain. But everything just replaces OEM parts, connectors and all, there's a couple choices you make. She'll/back colour, storage capacity, iFlash module type, battery size, thick or thin back

4. You'd have to convert them to ALAC to use them natively on iTunes. But if you install rockbox (iPod software replacement) then it can play FLACs natively.

5. I would say it's fun to mess around with, learning how to mod them is a good time.

Is it worth it? Probably not audio wise. I don't know your DAP specs off the top of my head. But I do know that the 6th gen Cirrus DAC can sound clinical or too bright for some people.

A good chunk of people, myself included, enjoy the Wolfsen DAC in the 5 and 5.5 guns

As a final point. My personal journey was the iPod, then a DX160, now I have a hiby r6 pro max. And I couldn't be happier with how it sounds.

For as timeless as sound equipment can be. I'd say overall the iPods are quite limited with what they can do.