Simple walker structure without submenu wrap and

Hello, I’m trying to make a simple navigation structure using the wp_nav_menu.
Already did the menu in the backend. The output looks like this:

<ul class="sitemap">
	<li id="sitemap-335">
		<a href="#"><h5>Sitemap-1</h5></a>
		<ul class="sub-menu">
			<li id="sitemap-333"><a href="#">point 1</a></li>
			<li id="sitemap-332"><a href="#">point 2</a></li>
		</ul>
	<li id="sitemap-336">
		<a href="#"><h5>Sitemap-2</h5></a>
		<ul class="sub-menu">
			<li id="sitemap-456"><a href="#">point 1</a></li>
		</ul>
	</li>
</ul>

(I removed unnecessary class names & links for clarity)

I want my output look like this:

<ul class="sitemap">
	<li id="sitemap-1">
		<h1>sitemap-1</h1>
		<a href="#">point 1</a>
		<a href="#">point 2</a>
	</li>
	<li id="sitemap-2">
		<h1>sitemap-2</h1>
		<a href="#">point 1</a>
	</li>
</ul>

I think I can accomplish this with a walker function, but unfortunately, I got no idea how to even start. I also read the documentation, but didn’t quiet get it right.

Can anyone help?