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.

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.

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