Stinky Overflow. Plug it?

Our bathroom sink overflow is driving me crazy with the awful smell.

I know it gets contaminated through regular drainage rather than overflow drainage. If I don’t clean it out at least once a week, foul smelling crud accumulates in the overflow slot.

I’ve changed the drain assembly 2x. I’ve disinfected it, used hospital-grade cleaners, hot water, vinegar. Nothing helps unless I clean the accumulated sludge, and I can’t figure out why it’s accumulating in the first place.

Should I plug the overflow slow? Tape over it? Is it the sink design that’s defective? I’m at a loss.