For heavy hashtag users, is it possible to hide hashtags from blocks but show them on mouse hover?

That’s a good idea! Unfortunately I couldn’t quite get it to work. However, by changing the width upon hover, I got something that seems to work OK. Is that close to what you wanted?

a.tag{
  visibility: hidden;
  width: 10px;
  white-space: nowrap;
}

a.tag::before {
  visibility: visible;
  content: "#";
}

a.tag:hover {
  visibility: visible;
  width: fit-content;
}

a.tag:hover::before {
  content: "";
}