Copy this into the custom Javascript section of your blog customization page, obviously replace with your own blog url and hashtag :
var a = document.querySelector('a[href="https://blog.echoingfrog.ca/short-stories"]'); if (a) { a.setAttribute('href', 'https://blog.echoingfrog.ca/tag:shortstories'); }
Create as many as you need/want. All the posts that include the hashtag will then be found under the subpage, and all subpages will appear as a menu on top of all your pages.
I didn’t create this, I simply copied from another blog (or I had found the solution here in the forum, can’t remember).
Hope that helps.