Storage
The first thing to consider when organising your music files is how will you store them?
I'll add a couple below along with what I use
On_Device
The simplest solution is to store your music on the device you are listening to it on. This means just storing the files on your mp3 player or computer. This is the cheapest and easiest method but can be problematic when adding or removing music as some 2000s era mp3s want you to use specific software to add music. These softwares were buggy at the time and most of them are unusable on modern windows, let alone support on macos. Even if you do get them running using linux wine or windows compatability layer, they can be super tedious to use. Even without these issues, moving the files between devices is still tedious requiring a snake of wires or dongle hell and it's often easier to download a new copy altogether
Cloud_/_NAS
I'm combining these as there isn't much difference once all is up and running but to explain breifly: A cloud server is a storage device accessed via the internet. Think Google Drive or Apple iCloudA NAS is a cloud server you own and run locally on your own network. The differences to bear in mind is that a cloud server usually requires a monthly cost, whereas a NAS has a lengthy and difficult setup and will very slightly increase your energy bills. Personally I use a NAS running nextcloud These methods allow you to stream music with a reduced monthly fee vs music streaming or download and upload any of your music files from anywhere with an internet connection as and when. The downsides to this method is your mp3 player needs to have internet connection, and there is a risk of downtime (which you will also have to diagnose and fix with a NAS)
External_Storage
Probably the best of both worlds from the previous options, External storage (sd card, usb stick, etc.) Alllows you to have the ease of the cloud options without an internet access. The main issues with external storage is compatibility, your mp3 player and computer must both be able to connect to the card or stick and you can very easily end up in dongle hell, especially with recent macbooks, if this isn't possible. The second is size. Loosing an sd card is super easy, especially with spring loaded docks like both my mp3 players have. You will also have to weigh up cost or fragmentation. You can either buy an expensive card upfront and have a ton of storage on it, or buy a smaller affordable card and buy another cheap one when you fill it then have to keep track whats on what