simple solution needed: how to listen to (digital) music in 2021

Andy and Matt: Unfortunately I don't understand most of that you are saying and I don't want to use a PC. So I guess that solution is too complicated for me. I still thank you very much for your ideas!

phase: that sounds interesting. Do you have a picture of your in use? I'd like to see which case you used and which buttons .... also: what will the display show? just text or also album art?

I understand completely. I don't like using my PC either. Spent months and about 1000GBP researching alternatives. I'm still listening to my PC now because everything else fell down on quality.
 
I've dabbled with batteries. My subwoofer and full range amps are powered by a 12V lifepo4 battery and pure sine wave inverter. My perception is that unwanted noise is reduced. I tried running my DACs from batteries but the high frequencies suffered, so I run them from the mains. I'd like to obtain some power conditioners for the DACs. I don't notice any difference when switching my media player between mains and batteries.

Would be interesting to know others findings.
 
I’m using a larger (1.2kW) mains isolation transformer and a CLCLC filter with a 15w load resistor after that.

I had attempted a clock upgrade to the SD card player board, but didn’t have a setup to assemble something that was 3mm long, containing 4 tiny solder pads. I think i saw it stuck to a piece of tape somewhere in my things, but can’t find it now.
I bought a fresh board after that and swapped over some power supply capacitors. An unknown brand and type of mlc bypass was surplussed from a clock power supply that was in a video device. Also used some better Nichicon parts on the last regulators in the series. Those changes made more difference than previous clock upgrades on other equipment.
Maybe someone will make a better NDK clock pre-mounted onto a typical package size board for upgrades however.
 
Maybe a tablet? Tablets that support USB OTG should recognize a USB sound interface (my Galaxy Tab A 8.0 and Acer Iconia B1-770 both will play music through an ADS Tech Instant Music RDX-150). Music could be stored on microSD cards (the Galaxy Tab handles up to 512 GB), local USB storage (using a hub), or from networked storage.
 
Some searching unearthed "USB Audio Player PRO" which includes a special USB driver for Android that allows higher sample and bit rates and stuff.
USB Audio Player PRO
They also have audio recorder and editing apps that work with USB audio interfaces.

Some other Android music players:
Rocket Player
Poweramp
Blackplayer
Neutron
NePlayer
Fiio Music
Onkyo HF Player