Nobody should ever buy a $1000 Atmos capable soundbar and feed it via plain stereo Bluetooth.  It’s a huge waste of money.  On top of that, you will probably have lip sync issues.  My advice:  If you are set on the Arc Ultra, find some way of getting HDMI-ARC into the Arc Ultra.  An Arcana or the Feintech device, or a long HDMI cable.  Otherwise, skip the Era 300s (again, Atmos is wasted via Bluetooth) and use 2 Era 100 via Bluetooth.  

As to the turntable, any Sonos device with a Line-In can be used.  Port, Amp, Five, Era 100/300+Line-In adapter.  

Thanks ​@jgatie 

Some questions please:

-my projector has a eArc output, so Arcana is pointless, right? all I'd need would be a  VERY long HDMI cable?

-does bluetooth connection work towards a PAIR of ERA?  stereo ? 

-Line in: can I directly plug the ProJect turntable to the line in or do I need a pre-amp in between? 

-would the PORT device be relevant in any ATMOS setup?

Again, many thanks 

1. Yes. 
2. Yes, stereo, and not any 5.1 formats.
3. If the ProJect has a built in pre-amp (sometimes the switch is below the platter), you would want to switch it to ‘Line’ instead of ‘Phono’, otherwise you’d need to insert your own pre-amp between the turntable and the Sonos. 
4. Not sure what ‘relevant’ means, but the Port is stereo, and not Atmos enabled. 

thank you ​@Airgetlam 

I am now convinced to find a way to have an HDMI cable between my eARC projector and the speakers.

Now my “new” issue is :

• if i get a Arc Ultra i cannot connect the turntable except through the optical/HDMI adapter but
  • 1/ I will have to switch HDMI depending if i’m listening to the TT or projector since there is only 1 hdmi in the ARC Ultra
  • 2/ Sonos support is telling me that the optical in is for TV audio signal and may not work for turntable signal (I have the issue with my old Playbar, no longer working through optical although it did before (see note below for details)
• If I get a pair of ERA I can connect the turntable to the line-in, happy days! But i cannot use them as a surround for the projector except in bluetooth (lipsync issues) or i’d need to buy another soundbar (Beam or Arc or whatever)
• Conclusion: if i want Atmos and turntable, only was is to have a pair of ERA with a soundbar and budget is an issue

What soundbar would be best ? Do you see any alternatives

I am very grateful for your help and support.

PS: my TT setup is : project TT +“Project e-phono Optical Box analogic/digital converter”, plugged into the optical in of my old playbar. Not working anymore, but it did work 1 month ago. Sonos support says format received through optical is not recognized as it is not “TV signal”

No, I don’t, although I wouldn’t connect the turntable to a soundbar or surround speaker directly in any case, it just isn’t designed for that. I’d get a separate Sonos device that has an analog line in, set that up as a second ‘room’, and connect the turntable to that, and ‘grouping’ the rooms as necessary for playback of the turntable. They’d play in sync for the turntable input, but be out of sync when grouped and  playing the projector, although some may not notice that.