I never noticed any latency when I’m not using bluetooth. And no, the devices do not speak to each other. For PA/pipewire, this is just an audio sink as any other.
There is latency when using bluetooth but this is pretty standard. It just doesn’t increase (or not noticeably) when streamed to another computer.
AFAIK Kodi can use pulseaudio and probably pipewire. I use Kodi too on those computers and I just leave it to use the default PA device that I’ve set. I switch the default devices with pasystray.
What’s usually breaking for me is paprefs. Every so often after an upgrade, the options are greyed out and I can’t share or access my devices over the network.
I never tried to setup simultaneous output before because I just switch from device one to another, but I just enabled it in paprefs and it’s working too.