You can use iTunes on Windows to load music on the stock firmware. If you load Rockbox onto it, it works like a standard MP3 player where all the organization is done at the filesystem level. Note that I don't really see a point in buying an iPod to load Rockbox onto because you can get lots of other standard MP3 players that aren't well over a decade old, such as the Hifi Walker H2. That also gets you other modern features like bluetooth audio, expandable storage, and USB-C charging. It really only makes sense to get an iPod if you use iTunes to manage your music.

If you're running the stock firmware, one other thing to keep in mind regarding storage mods is that the main limiting factor for how much music you can keep on an iPod is the number of songs, not the amount of storage you can cram into it. One optimization Apple used on the iPods was to keep the entire music database in RAM. This made the player a lot more responsive and more battery efficient, since you could browse through your library without needing to spin up or seek the hard drive. However, it also means that having too many songs will make the iPod unstable because it'll keep running out of RAM and crashing. The limit is around 20,000 songs on the 5th gen and 45,000 songs on the 6th/7th gens. This means that it's not really worth putting a crazy amount of storage in your iPod unless your library is all lossless.