Man, hate to burst your bubble but a corner brace under the back edge is not really what fixed the sag. Those kits are meant to hold corners at 90 degrees, not flatten a bowed top. Particle board that dipped half an inch is already crushed inside, so the brace is mostly just stopping it from getting worse. What you're feeling when you lean on it is the brace holding the back edge down, not the board magically going flat again. If it were me I'd check with a level in a month or two, because that dip usually creeps back. Still a solid fix for 12 bucks though.

Did you check the height with your hands hanging at your sides first? I went through this same thing a few years back when my dad watched me fight a board with a hand plane and told me my bench was fighting me back. Knuckle height is the old rule for hand work, and it lets you push from your legs and hips instead of leaning over with your shoulders. I dropped mine to just above my knuckles and my back stopped aching after long sanding sessions. It took me a week to get used to reaching down a bit, but it was worth it.