While they're not valid, I believe the browser will still respect non standard headings. The browser will treat them like non semantic elements (similar to a div), but you can kind of make it legit with a couple different aria attributes. I would recommend staying within the standard heading sizes though.
Hard disagree, this ain't gonna kill CMSs, what are you talking about?! Content management goes beyond just a website, especially in the headless cms world. Also those products can be as low as free.