Without seeing a horizontal view, I would first move the couch ahead as far as allowable and then use a clamping style wall mount, at 90deg. Like these
Or these
VideoSecu One Pair of Side Clamping Bookshelf Speaker Mounting Bracket with Swivel and Tilt for Large Surrounding Sound Speakers MS56B 3LH
https://www.amazon.com/dp/B000X9O8SI/ref=cm_sw_r_cp_api_Ut9zAb6RQQC5Z
Omnimount makes some nice ones too if you have threaded inserts.
The video secu stuff is great, even if slightly ugly. They also offer an omni mount style mount that works very well. I have speakers mounted to my ceiling using them.
https://www.amazon.com/gp/aw/d/B005BV0FHG/ref=psdcmw_297859_t2_B076BJTFVY
If you can get surrounds that match your LCR, that'd be best, I'd definitely get something large enough to provide enough spl without distortion based on your listening volume, considering the room is 6m wide, that'd put wall mounted surrounds at least 3m from the couch.
Let's say you regularly watch movies at - 15dB, that means you need 90dB of headroom. If the speakers in question are rated at 87dB 1w1m, that'd give you 97dB @100w. Since you'd only need 90dB, that'd cut the power requirements down to about 12wpc. A good deal of small speakers start to distort severely at even moderate power levels, despite their ability to handle more.
Since you mentioned elevation speakers, I'm assuming you have an Atmos setup. If so, I'd strongly advise against mounting the surrounds much higher than half the height of the room, otherwise the overhead sound will blur into the surrounds and vice versa.
With the couch against the back wall, rear surrounds are a waste of time. You'd be better served by adding a second pair of height speakers, either front height or top height.
