This projector, unlike many others, does do HDMI-eARC. But you'd need an extender that also does ARC. I've not seen a wirelees extender to do so. Some Googling brings me the Best 4K ARC HDMI Extender over HDBaseT for Home Theater (avaccess.com), that uses a network cable to transport HDMI-ARC, so you might get Atmos from a DD+ source. I've no experience with this gadget myself. 

Wireless HDMI transmitters/receivers do exist, but I’m not aware of any that also work for HDMI-ARC at a minimum, which is what you’d need for dolby atmos.  I’m not sure I would trust it to be reliable if you do find one.

You can easily get 5 meter (and longer) cables for HDMI-eARC.  I would really think about ways you can wire the setup, either temporarily when needed or hidden in some way, before looking at wireless options.

Thanks a lot @106rallye ans @melvimbe. 

Unfortunatly, I really can’t connect the projector and the Arc Ultra with a cable. It would have been so simple ;-)
Just to know, if I set the combo ERA300+ARC as a room on S2 and I use Airplay2, I understand that without an HDMI cable, I won't get Dolby Atmos, but will I at least get good quality surround sound? What I don't want is for the system to foolishly distribute exactly the same stream to all 3 speakers.

No, currently AirPlay 2 supplies a stereo signal to a Sonos speaker, no matter what type. You would not get a surround signal via AirPlay 2. 

My understanding is Apple has recently ‘opened’ up that format for non-Apple devices, but I believe Sonos hasn’t had an opportunity (or perhaps can’t, due to hardware constraints) implemented it. 

Thanks @Airgetlam. It seems that I got the ARC Ultra for no reason then 

This isn’t really a Sonos problem, but an issue with projectors in general.   You can’t get  atmos audio  from the back of the room to the front of the room without wires.  

You could maybe look into placing your source device in the front the room, along with the Arc Ultra so that your audio is wired, and the video is sent wirelessly to the projector.  There are products, like the HD Fury Arcana, that converts HDMI audio to HDMI eARC audio, and then pick whatever wireless HDMI works for you.  I’m not recommending this necessarily, bit it’s something to look into.

In general I would say that a projector setup isn’t going to work for you if you can’t have wires (whether it’s speaker wires or HDMI wires) between the front and back of the room, unless audio quality is irrelevant to you.