Shoto,
If I understand your question, this is really easy.
If you want to use a Pi as a streamer you can use Volumio to stream from a NAS or what I do is load my music files (~250 - 300 CDs) onto a microSD card and store them onboard the Pi along with the OS and Volumio image.
In the configuration shown, I’m using wireless networking.
You don’t need a Pi 4. I’m using a PiZero W2 which has more than enough power. A Pi 4 is a major overkill for this application. (I just checked my Pi and the average CPU loading is 14% with a system temp of 42’ C. It’s not even breaking into a sweat doing this!)
I use Samsung High Endurance SD cards so no worries about longevity. A 64gb card costs around $14 and I still have 30 gb of free space.
I can also stream radio stations from Volumio as well as stream audio from my phone or tablet via Airplay.
My setup today is a Pi Zero W2 => Topping E50 DAC (via microUSB to USB B cable) => Topping PA 5 amp => Elac Reference DBR62 speakers.
(That’s the Pi Zero W2 on top in the Flirc case.)
I typically control this from an iPad or iPhone. (I can do the same from my laptop but never do.) The Topping DAC has a remote as well to control volume and source.
Works great.
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