you can't set third party airplay speakers as default audio, only homepods
I'd highly recommend against using airplay if at all possible. it's picky about networks, and not the most reliable.
The experience will also vary from app to app. When using airplay, the remote speaker has a 1-2 second delay for buffering, airplay always has had this. Apple gets around this by delaying the video on the aTV to match the audio coming out of the speakers. Meaning every time you start playback, it has to play audio for 1-2 seconds before audio starts coming out of the speakers, and the video starts to play. Some apps deal with this better than others.
also I've seen debate, and never really a definitive answer as to if you get surround over airplay, or only stereo.
HDMI connections all around will give you the best experience, since the aTV will also be able to send volume command over the HDMI cable. as well as having everything turn on/off from the Siri Remote.
HDMI to the TV, then optical to the receiver will give you good quality audio. You should still be able to control volume and power, but it will take some work, as it will use the IR transmitter in the remote to send commands, and that can be a little fiddly to set up.