Hmm, 100, 200 and 300 all seem comically fast to me. Above 220 is annoying, or maybe it's my writing? ;-)

Because the Reader View is a protected page, I don't think add-ons can modify it, so you are limited in how you might approach this. But one option would be to use a bookmarklet -- a script you save to your bookmarks toolbar or menu. Clicking the bookmark executes the script in the context of the page. So for example, a script to set the max to 300 and the speed to 250 would look like this:

var s = document.querySelector('input.narrate-rate-input');
s.setAttribute('max', '300');
s.value = '250';

To convert that to a URL for a bookmark just requires a bit extra at the beginning and end:

javascript:var s = document.querySelector('input.narrate-rate-input'); s.setAttribute('max', '300'); s.value = '250'; void 0;

You can right-click a convenient spot on the Bookmarks Toolbar or Bookmarks menu, click New Bookmark, choose any name (for my test, I put "Fast Reader"), then paste the script into the Location field and save the bookmark. After loading a reader mode page, click the bookmark to make the change.

Thank you for a polite answer! It worked perfectly!