All my life I've been cavity prone. A few years ago I started swishing with salt 2x a day. Once in the morning, once at night. Also retrained myself to nose breathe when sleeping.

No new cavities in 4+ years.

The salt is extremely easy. Just take a like 1/4 of a teaspoon of salt crystals, swish gently in your mouth for about 30 seconds or so.

Salt has been well documented to kill cavity-causing bacteria: https://pmc.ncbi.nlm.nih.gov/articles/PMC7793524/