X bracing should help tremendously..and hang hose under or run down the top. And gussets on the hinge plate looked like the flat bar holding the hinges was flexing also. Best of luck. And great idea

Make the arm out of 1/2" ply. in the shape of a square tube. Make the 2nd section telescope into the first section. You will want to cut holes into the sides of the "tubes" to reduce the weight. But do it like a truss design. You can suspend the outer end of the first tube as you did with your design. That way you won't have the torsional twisting when you slide the inner section out of the outer section. You may have to design a little beefier hinge on the wall end, but the cable may be enough. I am about to build one of this design but mine will not be a long as yours. Mine will total between 8' and 10'. Yours looked to be about 12' to14'. Good luck

Add a chock to the bottom at a 45 deg that will pivot with ther arm...

Make the top or bottom piece of  solid wood 2 by material.

Use plastic tube with plywood braces. The weight is less due to the tubes

If you as chatGPT you will be told that a square is very bad at withstanding rotational force, You need something round for the first boom arm, either a plastic, fiberglass or wooden pipe, the wooden pipe you can make by gluing together 12 narrow boards all with 15° edges. ChatGPT can do calculations for you on the strength of your design. Good luck.

What kind of saw is that?