Bluesound node has digital in via USB and optical, and I believe it can read a NAS or USB drive directly without a computer. You can use that as a streamer and go RCA out to a fancy HP amplifier or just use the built-in HP amplifier on the node. If you’re willing to invest more money, you can go with a computer with Roon on it which will let you manage your NAS files and streaming (from Qobuz or Tidal) in the same interface. This will let you also access all that music on the go as well. Maybe as a next step upgrade down the line… :)

I don't own Synology NAS, but I have read some about it a while ago and understand what capabilities it has. IIRC you can install media server plugin to it and have it stream media files to upnp or dlna capable devices. I had a brief look at Synology's knowledge center on how to do that and I believe you need something in the middle if you need to connect to fiio k7 or k9. In simple term, you need a media renderer to output to your amp. A mobile phone would work or something like Raspberry Pi /SBC if you are a DIY type.

You can get a FIIO R7, and connect it to the same switch as your NAS.

I kinda discarded that has a lot of those IO would be paid for but not used. A bit too pricey

R7 is one of the cheaper options out there.

To connect to your NAS, you will need Ethernet. The Argon audio you listed, does not have ethernet, so it will not be able to connect with your NAS. Other cheaper options like ifi or topping M50 also does not have ethernet.

Network streamers with Ethernet aren't cheap. The most popular option is Blue node, which is $549 by itself. That does not include DAC or AMP.

Edit: Blue node does have headphones output, but R7 is more versatile in terms of headphones out.

Depends on your priorites.

Just add a look and the mini TRS in the front of the Node is an input. Can see it by the fact it has any kind of level control. My biggest doubt on the R7 would be how the HP amp stands against K7. The price gap is circa 500€, which is kinda crazy. At that point I could just power UP the computer and using foobar+K7 as usb dac/hp amp combo

If you have a spare phone or tablet, you can hook it up to K7 and stream your audio that way. For example in android I am using YAACC and UAPP app to stream from moodeaudio (audio streamer and player running MPD) to my phone.

Basically, after you install media server plugin on synology nas, it will show up on the apps I mentioned as UPnP/DLNA server. With the phone/tablet connected to K7 you can serve your audio files to it. Try this with your phone without having to buy K7 first. With an app like UAPP, you can access your NAS via ftp, sanba, or webdav interface and serve your files that way too.

The second part is true, the price gap is crazy for basically an extra streamer, but sadly Ethernet streamers are almost always on the pricy side. And technically you are paying for a small computer for it.

You could, like the other commenter said, use a spare phone and use a cheaper wireless streamer.