S87GMil said:
I don't work with code or scripts. I don't even know where I would put that. Surely we don't have to recode the site for ourselves for such a basic thing? I was hoping for an addition to the settings pages.That wouldn't work, because the code snipped above is JavaScript and the custom CSS Box can only be used to apply custom CSS styles.
The JavaScript code above can be executed in the browser's Developer console (F12), it would only change the destination of the 'nav-pools-link' element so it routes to "/pools".

The main problem with the code snipped is, that the changes made to the HTML Element will be lost after reloading or routing to another page.
So to answer the original question, there is no trivial way to change the destination of a link on the site.

S87GMil said:
there is no trivial way to change the destination of a link on the site.

// ==UserScript==
// @name         e621 Pool Link Modifier
// @match        https://e621.net/*
// ==/UserScript==
(function() {document.getElementById('nav-pools-link').href = '/pools'})();

Put into your choice of userscript manager.