If I have to write longer posted than 4,000 words, I break it into parts. For clients, I've found that due to some hosting environments and their limits, that trying to save 10K words on any post type causes error or the spinner just keeps spinning. I've not had this happen in Drupal as much, but for longer format. Interestingly, if I write a 10K+ chapter on FanfictionORG, then there's never a problem saving there.

Additionally, WordPress has a long way to go. It's not entirely web accessible. However, we're lucky to have so many thoughtful developers that give alternate ways we can choose to use the editor while writing content. A note - While most of my other newer sites use Gutenberg, my clients prefer Classic Editor, and I use the plain text (and have for many years) on my main website. For clients I write for, I write on Google Docs and send for them to edit, and then copy/paste into the WordPress editor.