I would love to hear your feedback on the (potential) differences between the input you use now vs. usb stick
I am not sure if I understand it right, but for the "Ethernet" input you would need a streaming service, right?
Not necessarily. You can "stream" your local files that you have on a PC, Hard Disk or NAS, for example.
You can do that with any UPNP/DLNA software as the Bartók is a UPNP renderer/player.
Two main ways to do that. One is to install a UPNP server software, this indexes your local music and makes it available for UPNP control points (like the dCS Mosaic app), that can then instruct the UPNP renderer (player) to go and pull that music file. Your player is the Bartók in this case. dCS recommends MinimServer as their preferred UPNP server software.
You can also use software that is both UPNP server and control point (and even renderer) all in one, like J River Media Center, for instance. In JRiver you can see and organize your music collection and "push" the playing duties to the Bartók as J River will see it in the network as a possible player, so you only need to select it as your output in JRiver instead of the local PC. You do all the music selection and queuing, etc. from JRiver that will act as a remote control for the Bartók.
The Bartók is also a Roon endpoint. Roon uses RAAT as the music transport/streaming protocol instead of UPNP. Roon is by far my favorite option because of all the great organization capacities of all your files, liner notes, artwork, relations between all the artists in different albums, etc. Roon has extensively tested the Bartók (and the remaining dCS stack, as well as several other vendors) so the Bartók is Roon certified. This means that when you select the Bartók as your player in Roon, it auto configures itself for the optimal settings in order to work with the Bartók. If you also subscribe to Tidal or Qobuz streaming, Roon is great because it serves as the single player for your local collection as well as those streaming services, so you can create your playlist with both local and remote files in a transparent fashion.
Roon is quite more expensive though, but I love it.
In any case, if you want to use the ethernet connection to play your existing collection, that is absolutely possible and there are some fantastic pieces of software, both free or paid that allow you to do that and have a much more pleasant interface.