@rimmi2002 Is that an acoustically transparent screen? Would you consider replacing it with one? From the photos, you have very limited space for tower speakers or stand mounts because of the doorway left of the screen and the inability to move them away from the wall. I am leaning with AcuDefTechGuy and thinking that in-wall or on-wall would be best if you want to keep the screen the same size, otherwise you may need to downsize the screen to leave room for speakers. An acoustically transparent screen lets you mount the speakers behind the screen, which not only hides them from view but you can place them at the proper height.
As to brands, I only have experience with Paradigm but given your budget I would be looking at Perlisten, RBH, Focal and KEF. Try and find impedance graphs if you can so that the impedance does not dip too low for your Onkyo, or consider a 3 channel amp to drive the front 3 (assuming your AVR has pre-outs). The
Perlisten in-walls would be a big upgrade from the Sierra 2.