For drifting singers, a compressor would be the piece of equipment that fixes that problem more the any microphone would would. You set it so that it sounds best at the quieter range of the singer and then when he/she jumps on the mic it will limit the signal and be more balanced...
https://www.bhphotovideo.com/c/produ...BI%3A514&smp=Y
read the third review on the link I posted which sounds like your situation