Move to the next level

@gelle with that type of budget my personal first choice would be the combination of the Chord Qutest and the Curious USB Cable for use with the Roon Nucleus. That is what I am using in my main rig. Having said that, your budget gives you plenty of other good options .... and a lot depends upon what you are looking for in terms of features from a DAC.
Other choices that I have considered (both new or used) include the very fine Ayre Codex and products offered by Schitt and Mytek among others. But as I said above a lot depends upon what features you value as well as the type of sound that you prefer. Also very much like the Bryston products. I have a BDA-1 (used to be in my main rig, now in a second system), but in your case you would require a BDA-2 or above to mate via USB with your Nucleus.

@gelle  what is your budget and what are you currently using for a USB Cable.

What I’m running with right now in my system would fall within your budget. Roon Nucleus feeding Chord Qutest DAC via Curious USB Cable. The Nucleus has a bay which can be fitted with an SSD or HDD such that it could function as your server.
At full retail:
Roon Nucleus $1399
Lifetime Roon Member ship
                         $699
Chord Qutest  $1695
Curious USB    $350
==================
Total                $4143
Still leaves room within your budget to purchase SSD and upgraded power supplies if so desired for Nucleus and/or Qutest.
I’m currently utilizing Small Green Computer 5v LPS ($160) with my Qutest and stock power supply with my Nucleus.
The key to all this is using Roon Nucleus as your Server, functioning as Roon Core Server and Player. You can substitute the DAC and USB Cable of your choice. I find that this setup works very well.

@gelle,
I think that if I was in your shoes I'd start with the DAC, but recognize that the USB Cable which is an important component in and of itself may be a limiting factor. It's all about synergy and everything matters. That being said, all things being equal, I'd start with the DAC, get it broken in and live with it for a while first before pursuing a better USB Cable. In other words, would rather match the USB Cable to the DAC, than working to match the DAC to the USB Cable. Where possible, I find it makes sense to control one variable at a time.     

@bruce19, I’ll attempt to answer your questions one at a time.
There is a fair amount of overlap when it comes to servers, streamers, network bridges, and players. But a fair amount of the functionality of the hardware pretty much comes down to the capabilities of the operating software that it is running. A server or player/renderer may or not have have an internal DAC. Most will also function as a network bridge. Most will also support use of an external DAC. A pure network bridge will require use of an external DAC.

Roon software supports Tidal, Qobuz, Internet Radio, and streaming from local libraries of Rips and Downloads. It currently does not support Spotify. What is simply marvelous about Roon is the way that it integrates selections from Tidal and/or Qobuz with your own personal local libary. Your Tidal and Qobuz favorites appear alongside your personal local stuff by Artist, just as if it was part of your local library. And what is also simply marvelous is the Artificial Intelligence built into Roon which supports music discovery. If you allow it, Roon will pick selections for you based on other selections you have played, both from your own library as well as from subscription services.

With Roon, you have the concept of Roon Core and Roon Endpoints.
Roon Core can be run on a PC or MAC, but to do it right, Roon Core really ought to be run on a dedicated Roon Server. A multitasking PC which is also running Roon is a recipe for sub optimal performance.

I picked up my Roon Nucleus in November. I simply love it and there is no going back. I’m currently streaming from my local library as well as Qobuz. Prior to Qobuz I was streaming from Tidal. And prior to Roon I was streaming via Bluesound products. I’ve had Bluesound kit since 2014 and have four pieces of Bluesound kit around the house, and all can function as Roon Endpoints. Via Blu OS I can get to other Blu OS supported subscription services such as Spotify (but currently I’m only doing Qobuz).

I had considered the Sonic Transporter, but considered the Nucleus to be a better solution as it is a perfectly tailored solution by the same vendor. Roon had long been on my roadmap. When my local dealer announced he was closing his store and retiring, I picked up the store’s demo unit at the retirement sale.

My system as of October 2019 can be seen here https://systems.audiogon.com/systems/8156
In November the Roon Nucleus replaced my Bluesound Node 2i as my principal streaming source.

I hope I have addressed your questions. If not you can either get back to me here and/or via PM.

In summary, Roon if done right is expensive, but in my opinion if you can afford it well worth it, especially in conjunction with Qobuz and/or Tidal.