I added a Denafrips Iris DDC to be able to take advantage of the i2S connection in my Musician Pegasus R2R DAC. I honestly didn’t go in with high expectations and bought the Iris used so I could easily resell it if the improvement proved marginal, but it was far from marginal. Adding the Iris was on the order of doing a complete DAC upgrade — the noise level dropped so images popped more out of nothingness and the 3D soundstage expanded in all directions. And yes, ideally the HDMI cable should be 1 foot or less as the i2S signal degrades quickly over distance because it was initially designed only for internal connections within a component. As I was just dipping my toe in I bought a $7 cable from Monoprice mainly because it was available in as little as 6” length and I wouldn’t be out much if I sold the Iris, but as it was more than capable of conveying the substantial benefits I heard I haven’t felt the need to upgrade to date. I’ve read several people say they didn’t experience big improvements from spending a lot more on an HDMI cable for i2S so that may be at play, but if I did upgrade it’d be to the DHLabs HDMI 2.1 that gets solid feedback but still not too pricey. Here are the links to both cables in case it helps.
https://www.monoprice.com/product?p_id=13578
https://silversonic.com/products/digital-cables/hdmi-2-1/
Anyway, that was my experience going this route and I know several others here expressed similar results so I’d definitely say it’s worth trying. Hope this helps, and best of luck.