Hi @TimShipston
According to the TV’s manual, which does not mention Atmos in any way, the best audio you’ll get from that model is DTS, where available.
However, some older TVs were somewhat restrictive in what they would pass from an HDMI input to the HDMI-ARC output - some would only give you surround sound if you used a source internal to the TV. That you can only get 5.1 using a TV app suggests to me that your TV is one of them, though I do recommend contacting Sony just to make sure, as the manuals do not list what a device cannot do, only what they can do.
I’ve had no further joy linking the TV’s arc socket with the Sonos ARC. but I’ve swapped over to the TV’s optical output and have now got most sources (TV apps, Sky Q apps, Apple TV apps) streaming in 5.1. Oddly, the only exception is Disney+ streamed via the TV app.
This is good - there is no loss of quality when comparing HDMI-ARC to Optical (when the TV doesn’t support Atmos, that is). The only real differences are that a) you can no longer use the TV’s remote to control volume until you program the Arc to accept the remote’s commands (see our Control Sonos home theater with your TV remote help page for instructions) and b) you can’t use a voice assistant on the Arc to turn on/off the TV.
To experience the full capability of your Arc, however, you would either need to replace the TV with a model that supports Atmos, or add a (unsupported by Sonos) device into the chain like a HD Fury Arcana (not a recommendation!), though I think these days you could get a new TV for the same price.
Glad to hear you are getting surround sound now, though!
"Common sense is the collection of prejudices acquired by age eighteen." - Albert Einstein