Tl;dr You can visually style your page structure like you want, but with HTML5 you don’t get to fiddle with your tags structure. Rules are rules and they’re simple.
H are for headings so any heading must be an H. No exception. Once again, it’s about accessibility, how pages appear with no style, how page appears with no screen, how page behave when navigated without a pointer device.
And accessibility is not meant—only—for people with a handicap. It’s meant for allowing anyone and anyTHING to access and read your content. Any technology, any machine, modern or ancient. Also, building accessible code is healthy for everything else, including SEO. It’s good for performances, robots, maintenance…
Why would we do that? h2 comes after h1, 3 after 2 etc. No reason to skip a level. No reason to consider that because a heading FEELS LIKE that other heading on another place of the page which is a h5, but is after a h2… No, you’ll still make it a h3, but you are completely free to VISUALLY STYLE it like that other h5 down the page. Once again, graphic design is disconnected from structure of content. A page can tell 2 stories.
Not sure what you mean here. H4 and 5 aren’t “small”, they’re just down the hierarchy. Use them when you should use them. If it means having a title for list items that are inside of h4 sections, then yes, bullet points TITLES will be h5. But not the bullet point itself, that’s a
<li>and only a
<li>(well you can put a p in it.)
They exist for accessibility, now. For most of the people, how you style them will still be more important. For robots/search engines, they’re less and less important. What’s important now is how your whole content design is achieved. We chase Google’s position zero nowadays, trying to make it clear for Google what’s an answer to a popular request. Information has to be precisely structured. Read @jmw post https://webflow.com/blog/how-to-structure-your-webflow-cms-collections, it’s gold with diamond sparkles. (And it will lead you to schema.org)