The iPad app will let you do exactly what you want, but you still need a Roon brain somewhere, we call it the “Core”. The iPad will just be a remote that does no library or endpoint management.
You would need 1 NUC or equivalent device for Roon. The RPi thing only gives you the endpoint, not that brain.
Unlike DLNA, our RoonSpeakers protocol allows endpoints are very lightweight, which is what you want near your audio renderer. If you make a DLNA endpoint that light, it’d be a WAV file streamer and not much else. We offload all the CPU and electrical noise in your system to 1 machine, which can be put far away from your audio gear.