Ceiling speakers are not a good choice for fronts. The should be in walls, flanking the screen or behing an acoustically transparent screen.
Using the suggestion by @jgatie you would need a second Sonos Amp to drive a pair of in-wall speakers on rear wall or in-ceiling speakers directly over the seating area.
Another option for rears would be Sonos One x 2 or Sonos One SL x 2. The former would give you a choice of Alexa or Google voice assistants. Last option would be your own passive speakers wired to the Amp.
FYI, a Sonos sub can be bonded to the Sonos Amp driving the front in-wall speakers or a 3rd party sub could be wired to the Amp.
If you are looking to setup a 7.1 system with left, center, right and surrounds placed one on either side wall and two on rear wall with a sub you’ll have to go with another system and not Sonos.
I’m bailing on the projectors. Going with a Sonos 5.1.2 setup using:
(1) Amp
(1) Arc
(1) Sub
(2) in-ceiling speakers as rear surrounds.
I’m bailing on the projectors. Going with a Sonos 5.1.2 setup using:
(1) Amp
(1) Arc
(1) Sub
(2) in-ceiling speakers as rear surrounds.
Since you said you wanted optimal setup, you should know that in-ceiling speakers are not ideal for rear surrounds. The audio should be coming from behind you, not above. That’s especially the case when using an Arc that up firing speakers with dolby atmos channels coming at the listening area from above.
If you can, I would use in wall speakers or sonos ones.