Sure, Oregairu has a manga. The manga expands on the story and characters from the original source material. It offers a different perspective and more detailed plotlines.
Overall, the OreGairu manga follows the light novel pretty well. There might be some minor differences in the way certain scenes are depicted or the emphasis on certain details, but the core story remains intact.
The Oregairu manga and light novel have their own unique features. The light novel is typically more text - heavy. It can go into great lengths about the philosophical musings of the characters. For instance, Hachiman's views on society and relationships are explored in a very detailed manner. On the other hand, the manga can sometimes take creative liberties with the story. It might emphasize certain comical moments more visually. The light novel may be better for those who enjoy a more immersive reading experience with detailed descriptions, while the manga is great for those who want a quicker, more visual take on the story.
I'd suggest starting from Volume 25 of the manga. It aligns well with where season 5 ends and takes the story forward smoothly. There might be a few differences in pacing compared to the anime, but it'll give you a seamless continuation.
It's hard to say for certain. Sometimes manga series continue based on popularity and demand. But for Chunibyo, we'd have to wait for official announcements.