llewark said:
Here's some CSS that may bring back small thumbnails and 3 columns on mobile.

I didn't test it, just got bored and poked around.

Apply in Settings->Advanced->Custom CSS:

article.thumbnail {
  img {
    max-width: 150px;
    max-height: 150px;
  }
}

section.posts-container {
  grid-template-columns: repeat(3, 1fr);
  --thumb-image-size: 150px;
}

It works and thank you