Wow, have been following this topic for a bit.
Have I not seen the mention of noise gates on all mic inputs?
A while back when I was running an analog rig (and poor) I built a 3U rack 8x compressor + noise gate.
The magic with the gates was not hard gating, but used a technique called an downward expander, which was like a soft gate. As you backed away from the mic the expansion increased until the mic was silent.
Plugged this into the inserts of all channels, and it worked a treat.
No feedback, and comments that asked if we were miming to a backing track (I eventually took that as a positive comment).
If you still get stage feedback, then the whole band is too loud, there is a maximum limit vocal monitors can go to, and with the rest of the band being too loud (can't help the drums) pushes the limits of your vocalist.
Most studio recordings don't have the band louder than the vocals, so try to keep your stage volume within reason, and let the front of house PA do yhe workout instead.