Whether you are using optical or hdmi, the minimum 75ms delay for grouped speakers will always be there for TV audio, only music streaming would play in sync across the group.

Edit: the “single rear speaker” is also just playing a stereo version of the TV audio, so not sure how that is enhancing your listening experience.

I think that only one rear as surround speaker isn't possible, I think that you're playing the TV sound in two different groups and with this the minimum delay is 75ms :)

The delay for grouped speakers with a TV source was always there. The only way the sync was perfect before, with either optical or HDMI, was if you adjusted the delay for the soundbar in the Settings for the room.  However, this can cause lip sync problems withe the video source.