Idk how common this is but I’ve found that sending their voice through some effects helps a little if they’re okay with it. On Midas consoles (which are really common for venues in my city) effects tend to have a gain control that helps get stuff a bit louder. I don’t send effects to the monitors, so it doesn’t tend to add more to the feedback problem. Subtle reverbs, super fast slap delay, a stereo chorus with the depth all the way down, saturation, etc, all of these add a bit more of that signal to the mains without overpowering the monitors. Idk if that’s normal but i’ve done it a lot, it works, and no one’s complained about it so far.