It was great.
You don't need to know about all the worlds (Darksouls123, Eldenring, Bloodborne, Dc, Marvel, Worm) but the more you know the more you'll appreciate.
The writing quality is one you would very rarely see in fanfics. Everything was connected, had a reason and plot points were subtly hinted, and the characters felt like themselves.
The girls and Mikael's relationship had a final look similar to other novels' harems but this one had very clear reasons and wasn't just the result of plot convenience, and their interactions felt natural and the characters felt real. The Yuri I don't typically enjoy, but I get now it was more about the improper portrayal I disliked. The girls weren't Mikael's. Nor each other's. Nor was this expected by anyone. They were just a family and got along some more than others.
It certainly isn't perfect and you'll probably need some patience in some parts and prior reading experience before you can fully enjoy it, but I recommend a try.