Open Graph tags using “Front Page” as title instead of my blog’s name

If you view the source on the front of bix.blog you can see it’s happening there.

Ah, I understand what you mean now! This should match the front page’s title, currently set to “Front Page”. If you edit the page title here, the Twitter tag will update too.

the og:title is set to the same as og:site_name instead of the page name, and og:description is set to the blog description not to an excerpt of the actual page. I assume that’s intended

It is intended indeed, although I could see how useful it would be to change that.

could I hook into the Jetpack OG function to use the page title and page excerpt there?

Yes, you can definitely change that, using the same filter I pointed out earlier. Something like this should work:

add_filter(
	'jetpack_open_graph_tags',
	function ( $tags ) {
		// Special condition for the posts page.
		if ( is_home() ) {
			unset( $tags['og:title'] );
			unset( $tags['og:description'] );
			
			// ID of our Posts page.
			$posts_page_id = (int) get_option( 'page_for_posts' );
			$tags['og:title'] = esc_html( get_the_title( $posts_page_id ) );
			$tags['og:description'] = 'a custom description just for the posts page';
		}
		return $tags;
	}
);