As far as I know, there's no hardware AirPlay transmitter available that can take audio input and send it to AirPlay speakers. It can be done with a computer and suitable software; and I believe someone built something like that for their own use with a Raspberry Pi and free software to act as a dedicated transmitter device, although it sounded like it wasn't the easiest to get everything together. I no longer have the link, because it seemed like something that would be too much bother (esp. a decent case including audio board; the fight with getting the right software, configured properly, I could maybe win, but making a case, not so much).
But here's the kicker: AirPlay (esp. AirPlay 2, which you'd need for e.g. stereo) has considerable latency, because it buffers a lot to be more resistant to variable network performance. It's enough that AirPlay 2 has lots of software hooks for video use, so as to delay the video the right amount to sync with the audio - and it makes pause, fast forward and so on, pretty annoying with their lag. For something like a piano, if you EVER want to play it manually, having the sound come out something like two seconds behind your playing would probably be so distracting as to be intolerable.
Bluetooth does not have this problem, and Bluetooth transmitters are readily available - and newer versions of Bluetooth can have pretty good range; I've walked over most of a small house wearing AirPods, and they held their connection just fine.