Looking for suggestions for a monster/human romance manga! I’d prefer that the male romantic interest is actually rather monstrous in appearance.

I’ve read (and enjoyed):
Mahoutsukai no Yome
Niehime to Kemono Ou
The Beast and his pet Highschool Girl
Itou-kun Doesn’t Know About Love
Totsukuni no Shoujo

For further reference, The Shape of Water is my favorite movie and Phantom of the Opera is my favorite musical lol

If you don't mind oneshots, there are a couple works by Nagabe that sort of has female human x animal-like creature relationships (same author as Totsukuni no Shoujo that you listed)

Azufareo no Sobayounin
Yuusha wa Shimei wo Wasureteru
Not a lot of chapters out for these right now, hopefully there will be more in the future.