For anyone else annoyed by this, I've created a userscipt to auto expand the textarea. It's not perfect, but it's better than nothing.

// ==UserScript==
// @name        Autoexpand textarea
// @namespace   Violentmonkey Scripts
// @match       https://p.feddit.uk/*
// @grant       none
// @version     1.0
// @author      flamingos-cant
// @description 06/08/2024, 21:05:31
// ==/UserScript==

function updateHeight(textarea) {
  var text = textarea.target.value;
  var lineHeight = 1.25 * parseFloat(getComputedStyle(document.documentElement).fontSize);
  var taHeight = textarea.target.scrollHeight - 16 * 2;
  var lines = Math.max(Math.ceil(taHeight / lineHeight), 8);
  textarea.target.rows = lines;
}

setInterval((function () {
  let textareas = document.getElementsByTagName("textarea");
  for (let textarea of textareas) {
    textarea.addEventListener("input", updateHeight);
  }
}), 2000);