The problem with swapping her shield regen from her primary to her secondary is you’re probably using your secondary at range and your primary up close. I get that you mean making it a flat 40% damage returned as HP (which could be a lot because 50 damage is splash damage), but chances are you’re not going to be in enough danger to really need the extra health vs. when you’re up close trying to pressure a tank.
having it on RMB can provide reliable sustain in close and long-range fights. allows for sym to be adaptable to various fighting scenarios and gives her opportunity to retreat with sustain. The current shield regen is barely noticeable and I do find myself using RMB in mid-close range at times too.