RTAudio isn't a DSP library, it's a thin wrapper around audio driver APIs on desktop operating systems (and maybe mobile? I haven't used it in years).

Elk is awesome but it's a proper linux distro, which may require more resources than the typical embedded audio device. But it's remarkable how much you can do these days.

I don't think JUCE belongs in the same conversation. It barely runs on Linux and the DSP library is not suitable for production applications (the IIR filter implementation requires allocation to set its parameters!)