Will this work?
This is WordPress themes specific, it probably won't work for normal websites... I'm not sure but you can give it a try.

.container {
  display: flex;
  flex-wrap: wrap; 
  box-sizing: border-box;
}

.item {
  flex: 1 1 300px;
  max-width: 100%;
  min-width: 0; /* You did this already, right? Keep it. */
  box-sizing: border-box;
}

.item * {
  max-width: 100%;
  overflow-wrap: break-word; /* The contents might be the one causing the problem? */
  word-break: break-word;
}

.item img { /* Are there images in your code or something similar? */
  max-width: 100%;
  height: auto;
  display: block;
}

.item .wp-block { /* Look out for WordPress blocks */
  margin-left: 0;
  margin-right: 0;
}