Sub-title HTML5 semantics?

Thank you for this plugin. My question is more a functional question than a technical, but I hope that you, or someone will help me with it.

I was thinking about a new posts title called “Staying or leaving (and other dilemma’s of choice)” when I thought that the latter part could well be something like a sub-title, keeping focus on the title while (like a book-title) still adding some sub-info to the title of the post. At that point I found your plugin and was pleased to learn that it is adapted to Gutenberg, but I found no answer on how a subtitle should best be implemented in the resulting HTML5.

After thinking about it, I am not yet sure about the semantics of having a subtitle, since it is translated in the heading structure of HTML, rather than being an independent title attribute. So in translating it in HTML5, aiming to use a semantic based structure like HTML5 seems to propose, I wondered what the target HTML should look like.

The title and sub-title are basically descriptive of the whole post, sort of a catchy entrance, so more or less equal in weight, just ordered in two parts. As far as I know HTML has no real provisions for that, just headings.

So should the title be an H1-tag and the subtitle an H2-tag and all headings below that H3 and below? It doesn’t feel right yet and I have learned SEO wants just one H1-heading per URL.

In book printing the headings are separate of the title and subtitle, so there it is not a problem, but here it seems different. Do you have any thoughts on how to HTML-structure the titles for optimal semantic structure (and seo)?

Right now I am thinking that the html should perhaps combine the two titles to one H1-tag and just visually change the parts to be presented as two headings with each different styling (the sub-title on a new line with smaller caps or something?). But that seems something of a tweak too, and is perhaps less than optimal.

Kind regards,
Hans