I fixed the audio issues I was having with my Chromecast (Google TV) by connecting it directly to the surround receiver. I really thought I tried that before, but looking up information of this problem made me think it should be the solution so I tried again and now it worked.

Weirdly I didn't get surround sound with Netflix or Amazon Prime, but I did with HBO Max with the same cables and devices. When running the Netflix or Amazon Prime apps on my oldish (~2016) tv, they did give surround sound. Nothing I changed in settings fixed this.

Turns out that Netflix etc provide Dolby Digital Plus signals instead of the normal Dolby Digital. The Chromecast is able to pass through either, but only if the tv can. The tv can only pass through Dolby Digital over its Audio Return Channel to the surround receiver. It tells as much to the chromecast, which tells the apps that only Dolby Digital, not Dolby Digital Plus, nor Atmos is supported. So far so good. It goes wrong when the Netflix and Amazon apps are like Dolby Digital Plus or bust and they fall back straight to stereo only. The HBO Max app correctly falls back to Dolby Digital. Somehow this only happens when Netflix / Amazon Prime run on hardware that theoretically could support DD+, because on the old tv they only try Dolby Digital and don't even think about trying DD+.

Anyway, great that it works now. Netflix, Amazon Prime, HBO Max and Disney+ are all in DD+. Leaves just SkyShowtime which has just two audio streams: one that's stereo and one that's 'completely freeze up the app'.