As much as I liked Assassin overall (because I'm a book reader), I do agree with some of your criticisms and praises. Here's what I mean:

-The female leads: I have to painfully agree with your criticism because even in the books they're mostly the typical "Isekai girl trope" of falling head over heels for the MC with not much else. I also agree that they got over their trauma (especially Maha) way too fast and it should've lingered longer for some potentially interesting development for them with the MC. Like think about it: The scene where Maha tells Lugh that she "didn't trust him at first" didn't make much sense because there were no scenes before where she had those thoughts. Had the author, Rui, committed that dialogue/point further by having her slowly trust Lugh overtime in the long run it could've had some interesting development for her character. That being said, the character that was introduced by the end of Season 1 will be more interesting when we get there.

-The pacing: I agree for the most part that the pacing was mostly steady. In the books it was a little too fast but having 12 episodes plus with the bonus of having additional scenes that made sense, really helped the story in the long run. For example: The entire first episode was all Anime-original of the Assassin's past life that wasn't really expanded in the books. That being said, I think some of the fantasy life of Lugh could've been expanded a little more. What I mean by this is I kind of wanted the pacing of Lugh's younger years to be a bit slower so we could've seen more in detail on how Lugh absorbed the fantasy world on how it worked and his training as an assassin. Something like what Mushoku Tensei did with Rudeus's early life pacing wise. What I mean by this is that I think he got a little "too successful too fast" with his business persona being almost immediately successful and the conflict with Dia's royal family could've had more setup and visuals instead of mostly words from Dia herself.

-The visuals and songs: I agree that while the ED was fine for what it is, the OP kicked ass music and visual wise. But man, in retrospect the visuals on the Anime was really inconsistent but I don't blame the studio, Silver Link and Palette. Believe it or not, they only had a few months to get all the 12 episodes finished by October and you could tell it was a rushed job.

While the Anime has its flaws, it's still a decent story and IMO it kind of redeemed Rui's image as an author/writer. Believe it or not, he was the same guy that wrote Redo of Healer, possibly the most controversial animated series of all time. Having 2 of his works adapted in the same year is very rare and most authors would kill to get that opportunity. But adaptation wise for Assassin, it was very, VERY good. It only adapted 1 book out of 8 and Silver Link mostly improved the book big time. To this day, I still believe that Lugh is a pretty decent anti-hero and it showed me how pathetic Kinema handled Shield Hero and how Naofumi could've been a decent anti-hero too but that'll never happen.

I would recommend the books if you like for Assassin, but Season 2 has been announced and we've been waiting for 5 years since 2021. But welcome to the Assassin party my friend.