Yes, I was trying to keep matters simple, as to what you will (most probably) actually be able to get from your TV with its Arc Port. The Arc is capable of handling Dolby 5.1.2 audio, but it’s likely the TV with Arc may only output Dolby Digital or Dolby Digital Plus 5.1… probably with/without atmos metadata. The two things you won’t get however, is uncompressed Dolby TrueHD or Multichannel LPCM, as both those codecs require an eARC port connection. So the Sonos Arc will play all other formats with a connection using an HDMI-Arc port.
Most users set their TV output to Auto, or Dolby Digital Plus. If ‘DD+’ isn’t supported by the TV then select Dolby Digital 5.0/5.1 - NB: the Arc will ‘merge’ .1 LFE channel using the mid-range drivers, although I would also recommend you consider adding a sub and the two rear surrounds to the Arc, at a later time perhaps.. maybe moreso than upgrading the TV.
If your TV (streaming) Apps support Atmos, you will get compressed Atmos with DD+ too, but usually the TV may not have those Apps (this usually depends on the age of the TV) - the TV may not support pass-through of the codecs aswell in some instances, but there are products on the market to get around some of the older TV limitations, such as HD Fury’s Arcana (for eARC uncompressed atmos audio capabilities) or even some much cheaper audio switcher/splitters for compressed atmos audio. If you need to read more about those devices then see these links:
https://www.hdfury.uk/product/hdfury-arcana/
And just as an example…
https://www.amazon.co.uk/dp/B07PS4JGX1
Hope that assists further.