With I2S how much does the streamer hardware matter?

Yes the streamer matters.  I’d keep the Lumin and add a DDC like the Denafrips Iris, Musician Pisces, etc. that will allow you to use the i2S input on your DAC.  Another Pontus ll owner here added the Iris and realized significant improvements in sound quality.  Hope this helps, and best of luck. 

i2s is but a connection type for digital music data... what goes through the connection still matters... so why would a good or bad streamer up front matter any less or more?

and yes -- of course, the proof is in the listening, always...

@calieng I also own a Denafrips DAC (Pontus II) and an Iris DDC, and recently bought a Mercury V2 Streamer. I was similarly impressed. The two streaming paths I compared were:

1. Roon -(ethernet)-> RPI4 -(USB)-> Iris -(I2S)-> Pontus
   and;
2. Roon -(ethernet)-> Mercury -(I2S)-> Pontus
   NOTE:  I have not tried anything like:
3. Roon -(ethernet)-> Lumin -(USB)-> Iris -(I2S)-> Pontus

I can say that, similar to your situation, I thought there was a noticeable improvement going from #1 to #2. I found the Mercury's low end sonics were similar to the RPI4 -- perhaps with slightly more defined attack; but there was a noticeable difference in the high end, particularly the attack / decay on percussion such as sticks, as well as better low level detail in woodwind instruments, faint cymbal swells, and other sounds with high-frequency components. Everything just sounded better, cleaner. 

Why? I suspect it is because USB in isochronous mode (which is used for audio transport) is a somewhat unpredictable transport (e.g., there are a lot of complicated calculations going on with the EHCI -- Enhanced Host Controller Interface -- which depend on the operating system, among other things, not just the hardware). On the other hand, I2S uses the system clock -- pretty much a direct connection to a crystal oscillator. Because of this, I think the direct to I2S configuration of the Mercury, delivers almost all, if not all, sound data without any timing problems; this precision most likely leads to the increased sonic resolution.

It's easy to believe a DDC can reclock a signal more accurately (assuming its oscillator is higher quality), provided that no packets are lost in transit to the DDC. However, the USB protocol makes that guarantee of no lost packets difficult to achieve. If packets are lost or late, the DDC can't help.

Can something like the Lumin connected to the Iris sound better? It's possible the Lumin has a great USB interface, although I suspect that, like most manufacturers, they are buying the operating system from elsewhere (or using Linux for free). Perhaps you can try the Lumin through the Iris and let us know...

It seems the Mercury I2S is about as detailed as the Lumin U1 Mini thru its USB into a Denafrips although it seems a little sharper sounding. Have not compared it to the full U1 yet. 

One drawback I did not realize is that it is very limited to formats. I do not see any DSD and for PCM nothing over 192kbs. Maybe I am missing something? I have it setup with the vendors version of RopieeeXL and using Roon. The Lumin on the other hand can do DSD 2.8Mbs upscaled audio.