How bad is it to place 5.1 rears from behind rather than on the sides?

I’m building a new setup and planned placing my 5.1 rears in-wall directly on the sides and perhaps pushing the sofa forward a bit while watching a movie. I especially wanted inwalls to avoid my youngsters messing with them and bought the Morel xbw600 two days ago for that. But renovating now, I see the only option to make them in-wall is behind me in 90 degrees (separated up to 3 meters apart).

So I’m facing a dilemma: do I place them behind or do I return them and get satellites instead that I’ll place on the sides (1.5 meters from me in each direction)? What do you smart people advise?

Illustration: https://ibb.co/86wc61J