Todo's appearances in the JJK manga are scattered. It depends on the specific storylines and conflicts. Sometimes he's in the foreground, and other times he makes cameos to add to the overall narrative.
Well, one method is to check out fanfic review blogs or websites. Some bloggers dedicated to JJK will review and rank fanfics. Also, ask other JJK fans directly. You can go to anime or manga conventions where there are likely to be JJK fans. They might be able to point you to some amazing fanfics they've read. Additionally, explore the related tags on fanfic platforms. Sometimes, the less popular but really good fanfics can be found this way.
I'd recommend beginning at the beginning, so Volume 1. That way, you get the full story as the author intended without missing any crucial details or build-up.
I'd suggest beginning with the first chapter. It sets the foundation and introduces the main characters and the initial plot. From there, you'll be able to follow the story's progression smoothly. Also, check out some online manga platforms that have clear starting points and good reading interfaces.