Streaming services don't use Dolby True HD, they only use the lossy version of Atmos over Dolby Digital Plus. The ONLY way to get True HD lossless is via disc (or certain files on a local media), so connecting your Apple TV via ARC won't lose you anything.I think my LG OLED65C7V is supposed to support Dolby Atmos (DD+/Lossy version) over ARC but it's not working using the built-in Netflix app and I've read numerous people having similar issues online.
I would loose out on HD-formats completely by going through the TV over ARC. With the Apple TV 4K connected directly to the soundbar I get Dolby True HD and DTS: Master Audio decoded into LPCM when using Infuse. This won't be possible over ARC as ARC does not support lossless formats at all, you'll need eARC and HDMI2.1 for this and that's only available on the newly announced LG 9-series for OLED's and the Samsung HW-N960 does not have eARC/HDMI2.1 so it wouldn't work even with support on the TV.
As far as Atmos over DD+ not working on your LG I would check around on the LG threads, it's probably a setting you're overlooking (LG has a lot of ARC settings and such disabled by default and you have to manually turn them on).