If you'd like to try this script:
1. Install a userscript manager https://greasyfork.org/en/help/installing-user-scripts (I use TamperMonkey https://www.tampermonkey.net/)
2. Install AO3 Floating Comment Box from this page: https://greasyfork.org/en/scripts/499115-ao3-floating-comment-box
3. Go read a fic. In the top left corner, there should be a little blue box with an O in it.
4. Click the little blue box. It turns into a little red box with an X in it, and a comment box opens at the bottom of the page.
5. Drag over some text in the fic.
6. In the floating comment box, select [Insert Selection]. The selection is added to the floating comment box, with HTML tags around it.
7. Type your comment below the quote.
8. Keep adding more quotes and comments until you've finished reading.
9. Optional: you can toggle the comment box on and off with the little icon in the top left, and it retains what you've quoted & written so far.
10. When you're ready to post the comment, select [Add to Comment Box] in the floating comment box. Your quotes & comments get pasted into the main comment box.
11. On the AO3 page's comment box, select [Comment].
When I'm reading a chaptered fic, I like to read chapter by chapter, and leave comments as I go.