I would probably remove the ones holding up the controller and maybe the Z axis supports. They don’t look so bad on the corners. But they are weird in the middle.
I have some corner brackets like this. I have two kinds. Mine came from a kit with 10m of 2020. The black ones were a little longer and have extra tabs. But they interfered in two places, so I also bought the more standard sized silver ones from amazon:
https://www.amazon.com/gp/aw/d/B07GGLYX9V?psc=1&ref=ppx_pop_mob_b_asin_title
I bet you could get away with those for the middle pieces. On the outermost corners, you probably still need plates. An arc like your 120 logo would be great. Except it needs to be 90 
. Even two plates per side is probably enough racking prevention. I wonder if you could quickly cut them from ACM. Mine are 3D printed.