Hey everyone - I would like to add my experience to the conversation as well and get some feedback if possible. I'll refer to my OLD setup and my NEW setup below.
OLD setup:
2009 Mac Mini running iTunes (iTunes version 12.7.9.5)
Apple TV 2nd Generation (AppleTV 6.2.1 - the last supported os for that generation)
Hooked up to Onkyo receiver via optical cable
Yamaha RXA3060 receiver with Airplay 1 built in
Apple HomePod (iOS 11.4)
NEW setup:
iPhone running (Apple) Music App
(2) Apple TV 4Ks (TvOS 11.4)
(1) Connected via HDMI to Yamaha RXA3060 receiver
(1) Connected via HDMI to Sony LCD TV
Apple HomePod (iOS 11.4)
My OLD setup works pretty well. I use iTunes on the Mac Mini as the audio source and Airplay to stream to all three devices. I can control the playback from my iPhone with the (horribly outdated) iTunes remote app.
The audio is very much synchronized. I will not say it's 100% perfect, but it is good enough for my needs. Surprisingly, the HomePod and the Yamaha are the most in sync (which is good because they are closest to each other).
My NEW setup is unusable. There is 500ms - 750ms latency between the HomePod and the Apple TVs. The latency between the two Apple TVs is manageable (< 500ms, probably more like 100ms - still not as good as the OLD setup).
This is consistent regardless of the audio source: iPhone X, either Apple TV, HomePod, MacMini (using Airplay 1 multi room audio).
Unfortunately, I've invested a lot of money and time in 'upgrading' to Airplay 2 and I'm worse off than I was with old (and in some cases obsolete) hardware / software.
As a last ditch effort, I enrolled one of the Apple TV 4Ks in the beta testing program. It is now running TvOS 11.4.1. this has made no difference.