There’s no simple/ideal solution. As you say, you need a soundbar to get the tv audio. That will give you the three front channels. The sound should be better than from the tv speakers, but possibly at the risk of a narrow soundstage. Separate left/right speakers placed wider will improve that and could be achieved using Amp and two dumb speakers.

Unfortunately, grouping your current speakers with the (whichever you go for) tv sound player (Amp, Ray, Beam or ARC) will have approx 70msec sound difference due to the way Sonos works. 

Thanks for your input.  Do you know what would happen if I ran a 3.5mm cable from the back of the TV into a line-in Sonos product (Amp, Era, Port, etc), and then connected the other speakers via the app - would that still result in the 70ms delay?

Yes. In order to achieve synchronised sound, Sonos sets up a delay - a buffer of time - so that any dropped packets of data can be resent to a speaker without the music stream stuttering or dropping out. When it’s just audio, there’s nothing to relate it to, and you don’t notice the delay when you start streaming music. It’s only when you have video to relate to that you see lipsync imbalance. So, the delay is minimised for tv/video audio into soundbars, but it still exists when that audio is sent to grouped additional speakers. 

Some folk on here have said the lipsync isn’t an issue for them. You could always buy the speaker and try it out in situ. If it works for you, all is good. If not, return it during the home trial period for a refund. 

So am I right in thinking: if I purchased a sound bar, the sound bar audio would be in-sync with TV picture but out of sync with any additionally grouped speakers.   But then again, sound bar audio may be enough (but a shame not to utilise, e.g. the mini sub). )Edit- actually reading this it makes no sense that sound bar & additionals would be out of sync.  So I guess grouping the additionals will actually throw the sound bar out of sync to the TV as well?, whereas without the additionals it would stay in sync?]

And if I purchased an Era, the audio from Era & additionally grouped would be in sync together, but all out of sync with TV picture?  In which case, as you suggest it may be worth buying an Era just to see if I can live with 70ms lipsync delay.  

The soundbar will stay with the video signal. If you then group the additional speakers, they will have a delay whilst the soundbar remains timed to the video and hence the echo effect from the addition speakers. 

Gotya.  Thanks for the clarification!