You could keep what you have and get a Raspberry Pi to act as music server if you wanted. I imagine the old Synology is fine if you’re just serving files.
There are loads of guides to setting up Raspberry Pis, and I find it quite fun. If you don’t fancy that, there are packaged installs available for Squeezebox Server that are almost plug and play.
I tend to try to keep stuff until it really is beyond serviceable. I’m not sure this is always the best idea, but I do loathe binning stuff that works.
I got a Synology 218J some years ago from scan.co.uk. mine is the unpopulated version so I had to bung in a couple of drives. Can’t recall what they are - probably WD of some flavour.
The whole thing has been very reliable and was easy to set up. I use it to stream flac files and video. It just sits in the study and does it’s thing without fuss. It even updates itself on its own.
Why scan.co.uk? I live nearby and they are handy. I imagine buy from anywhere you fancy really.
Why a Synology? Somebody on another place (the Big Tent) recommended them and did a “how to” “back of a fag packet guide” and he used a Synology so I just followed suit. Was that Gyroscope? Whoever it was, ta.
Music is not high workload for it. Video is a different animal. I’d expect most NAS kit that can do video to easily manage music streaming. The original PIs could do it and your NAS probably has at least that much power.
The workload for streaming music or video is actually very similar unless you start transcoding which has a much higher overhead for video than audio codecs.
I use it for the BBC Sounds app - I listen a lot to BBC catch up. I like being able to control this on the tablet and sounds great through the Modwright Transporter. Everything else plays via Roon.