You can create a small embedded component to all the templates which will print the page title / a page property as an H1. You can write a CSS rule to hide this H1 to be visible on the page. This way you can bring an H1 to all the pages including existing pages.
SEO also advises not to have more than one H1 per page. If that's applicable for you, then we can convert the existing H1- H6 selector to push a <p class="h1"></p> instead of <h1></h1> when they select H1 from the dropdown selector.