Better coordinate the depth of the Mantle and the various positioning of the moving parts of the movable bracket.
You might need to add some blocks behind the bracket to push the monitor/brackets out to make it work.
Buy the most heavy duty bracket, I put a common one in our Apt in NYC. Pulls out a bit, then angle it sideways to sofa. Straight when pushed against the wall, pull it out, it’s not quite level.
The bracket holes on the monitor are centered, but the weight of the monitor is not 50/50, so the off center weight of the monitor and the slightly flexible pivot joints of the bracket when extended let the right side sag a little bit. Many don't notice, I am an Interior Architect, have a good eye for level, pi---s me off!
And if monitor lowers, you need more clearance down to top of center.
I used this Bose Center Speaker for years, sounds surprisingly good, smaller, less clearance needed below monitor
or center speaker lower, angled up as someone suggested.
Has anyone used the TV’s speakers for Center Channel?