I'd say I love fan content Douman while not feeling much towards canon Douman.

First of all, in chapters until Heian, he kept boring me with the most cliche villain speeches, and to make me bored is the biggest sin a character can commit. I just don't find his caricature failvillain state amusing or entertaining to watch. Secondly, Heian chapter is just a chapter I couldn't connect much with. I kept falling asleep during the whole thing. Like, physically.
And Douman having a few cool scenes by the end sadly was too little to save the whole thing for me.

However! I like the concept. I love thinking about the idea that Douman was a normal person warped just by the perception of him as Seimei's antagonist. I love Seimei knowing the good of him while he envies Seimei being loved by the whole world. I'd love to see more of -that-.
That's where the fan content steps in, biting into the actual meat and showcasing all the interesting drama.

Douman has a lot of fan content filled with creativity and good storytelling that left more impression on me than his actual canon appearance.

The absolute crown of Douman fan content, I think, goes to this doujin about Douman becoming Sakura's Servant.
Part 1: https://www.pixiv.net/artworks/91558871
Part 2: https://www.pixiv.net/artworks/91559210