Hi, Gavin. If it is only the transport you want to chnge, I have been using a Lumin U1-Mini for a little over a year and it’s been a delight. I use it with a network connection in and USB out into a Meridian 818v3 mainly to play DSD format natively and 176+ PCMs. Soundwise it is very close to direct streaming via the ID41 card in the 818v3 when comparing streaming at the same resolution; I have a very small preference for the ID41 but that may be because I am more used to it or Placebo or it could be because feeding through the Meridian naturally favours synergies with the proprietary component! If you like your Benchmark DAC (and presumably there are synergies with the rest of the stack), I am sure the U1 Mini will work very well as a transport without needing to pay for an extra unused DAC. I chose Lumin over other transport alternatives because they are well established and with a good dealer network and support and I don’t regret it. The U1 caps at 256 DSD but if you don’t upsample and keep your existing it will not matter and still be largely future proof.
A big plus is that if you have any issue that can be addressed online, Peter always answers and helps whatever serious forum you post on. That is really outstanding service and he is by definition more knowledgeable than any dealer and will usually allow you to address any issue quickly.
Integration with Roon and MQA is seamless. The Lumin app is good and reliable (you only need it for setup if you use Roon); I have an issue using the Lumin app from Android (from my Samsung S10+) but on iOS always fine. If you want to use the Lumin app you need to be able to install Minimserver for access to your music on your network. If you have a large music library, the process to update it when you add new music isn’t the fastest but none of this matters if you are using Roon. Having an alternative is always a good thing nevertheless.
The compact form will also go well with your Benchmark stack! The power supply is integrated which my be an issue if you like tinkering with that sort of stuff (there are mods available I believe although they probably invalidate your warranty). Note that the back is designed to partially hide cables but also means that some proprietary power cables with a very large connection won’t fit.