I was a believer in i2s fairly early on and on paper it makes sense, however.......
After trying both PS Audio and Topping DAC's with i2s inputs, connected to either a RPi streamer or NuPrime CD transport with i2s outputs, the results were not show stoppers and in all combinations a decent (Only £50) coax lead sounded better using SPDIF!
As in everything, its all about implementation and for what ever reason, i2s just didn't work for me using these items, so be aware that improvements using i2s is not a given. The HDMI i2s pin outs were compatible and correct, as indicated by the manufacturers diagrams, but it only ever sounded ok, even with a short but expensive HDMI lead, supposedly designed for such a connection.
I now use 75ohm coax with BNC connectors all round and I'm happy with that arrangement.
I use an Eversolo DMP A-6 ME with LPS upgrade and BNC digital in/out conversion.
This Streaming DAC is very user friendly and sounds great. In my opinion, you would have to have a very good quality and costly set up to hear any benefit from a more expensive Streamer/DAC. My set up is a Pass XP-12 Pre into Neurochrome 686 Monoblocks with balanced XLR connections throughout.
I try to keep my box count to sane levels and feel certain combinations go well together. A Streamer/DAC combo is a natural choice in my opinion as is a Pre Amp/Phono stage, but having said that, I have the Pass XP-17 which is a separate phono stage. The one thing I don't want is a Streamer/DAC/Pre combo, as this is invariably a digital Pre Amp and the built in analogue to digital conversion, needed for my phono stage will be a joke.
My only complaint with the Eversolo A-6 is its only available in Black, other than that, it's a keeper until the next leap in technology, provides even better quality/features at a reasonable price.
God luck with your system building, I hope you find the synergy we all seek in this hobby, strike that, obsession!