Can you recommend some good doujin novels?

I'd suggest 'La Blue Girl'. It has an interesting plot and detailed illustrations. But keep in mind that hentai manga often contain adult content.

Yes, they are. In the manga, Yui and Sae are depicted as sisters.

The One Punch Man anime is known for its close adaptation of the manga. It retains the main plotlines and character developments. Some added details or slight changes are made to make it visually appealing and fit the animation format, but they don't stray far from the manga's essence.

I'd recommend 'The God of High School'. It has a similar action-packed and fantasy-filled storyline as Solo Leveling.

I'm not really sure. Maybe there's a scene in the manhwa where a housekeeper uses a neck pillow for comfort or something.

I have no idea about such a comic. It might be something inappropriate or not well-known.

They often have exciting sci-fi elements like space exploration and unknown creatures. The art is usually detailed and immersive.

There are various storylines where they face off or interact. One common one involves Dr. Klaw's attempts to obtain powerful resources and Black Panther's efforts to stop him.

Yes, they are. The boys seem to be still very much into the comics.