The HomePods achieve this because the “Default Audio Output” method (which require that both devices have to be in the same room within the home app) is not really AirPlay, but an ad-hoc dedicated network between them and the Apple TV created via AWDL (Apple Wireless Direct Link). That allows for real time audio to be transmited with no latency and surround sound support, including Atmos.

If you connect the Homepods trough the Apple TV control center they use Airplay and therefore have the same limitations that any Airplay speaker out there.

This should be fixed in the lastest beta (beta 3). Are you completely sure you are on the latest release?.

Two reasons.

A). A surprising amount of SDR apps do not flag their video output correctly when the Apple TV UI is set to HDR/DV, and therefore, either they do not switch dynamic range properly, or they do not map out correctly their video/color levels within an HDR signal/container.

B). Perhaps more importantly, the tvOS interface is not, I repeat, is not, natively rendered in DV. Therefore, when you set the default Apple TV video output to DV, a conversion from SDR video levels happens, which affects both color and accuracy.

In sum, set your Apple TV to 4K SDR 60hz, RGB High (to avoid the green tint bug with SDR content when set to the default YCBCR), and enable both range and frame rate matching. That’s it.

so Infuse holds no real benefit for me.

The tvOS Plex app currently has a massive stack of issues, including a critical one that overheats the third gen Apple TV 4K with high bitrate 4K remuxes to the point that it chokes on them and stop playing (I´m not joking!).

Infuse does not overheat it because it uses the Apple TV hardware acceleration for every aspect of their custom player (Metal), whereas Plex still uses Open GL for quite a lot.

My recommendation at this point is to use Infuse instead. Yeah, its not free, but it uses the Apple TV hardware and feature set to its full potential, and performs so much better.