I remember how conflicted things were when you landed on the SuperUniti due to delivery delays, hardware problems, and feature changes in the released new Uniti platform from both the old one and the advance marketing photos. It’s great that your choice of the SU has served you well for these years, and your desire to move again is happening only now that even newer things are coming out.
As you may recall, I did move from SU to Nova back then… and for me, like a couple of others in this thread, it has been a solid and happy move. The streaming features work well, and I easily enjoy both the Naim App and the native Roon Endpoint control routes. Lots to be said for this “natural progression” path… but not if you are still less than pleased with the Naim app.
I also recently bought an RME ADI DAC FS as a separate headphone station. I feed it from a RPi4/RoPieee streamer and it is amazing.
Given that you have already purchased the Innuos Zen streamer, my suggestion if you go this route would be to just add a power amp and use the RME as a pre as well as a DAC. No need for doubling up on everything, as you say. I think you could try this out into the SU as power amp (but not sure anymore if there is an input that bypasses the pre?) before you start choosing…
But this path really takes you away from Naim, I think. Not that there’s anything wrong with that, of course!!
It also leaves you without an analog input for future vinyl expansion. The Nova is great in this regard (I just added a TT to my main setup; the digital preamp in the Nova uses/requires an ADC on the analog inputs)… but the studio provenance of the RME Pro DAC, which has an amazing ADC side as well, unlike the DAC-FS, might be something to consider if you are going totally outside the box.
If you’re sold on the Zen, or if it’s beyond the return / refund date, my advice is to optimize your next move based on your new streamer.
If you already know you want the RME, just add power (it’s a good pre) or even powered monitors.
If you want to add an external DAC only, maybe look elsewhere than the RME and also look to a Naim integrated like the SuperNait3 (which addresses your future plans for phono at the same time). With this latter setup, you could choose a cheaper DAC-only unit to get started (there are lots to choose from) and free funds for the new SN3 integrated with the SU trade-in sooner rather than later. My expectation (based on reading, not a demo) is that the SN3 would be a decent amplifier upgrade over the Nova in the same way that the Nova improves on the SU. There’s more to these new models than just feature tweaks …
Food for thought, and best wishes as you navigate your path forward. Exciting times, enjoy the journey as well as the destination.
ps - The extent of Naim app / firmware issues often seems to me like it might be worse on the forum due to the nature of the interactions here, but there is always a risk that you could get bitten yourself, which is frustrating and painful indeed… the beta program is extremely useful as a place to go for early solutions and to highlight any issues that you are facing in your use case… a gentle recommendation to revisit that concern one last time before you really step away.