There are a few options. You could look for specialized manga download platforms, but make sure they are legal and trustworthy. Sometimes, manga artists and publishers offer official raw downloads on their websites.
Jump Plus manga encompasses a wide spectrum. Whether a manga is seinen within Jump Plus depends on its specific content and intended readership. There's no strict rule that everything on Jump Plus is of the seinen category.
Sure, Manga Plus by Shueisha is legal. It operates within the framework of copyright laws and partnerships with the creators and publishers to bring manga to fans in a lawful manner.
Sure, Manga Plus is a legitimate option. It provides a legitimate and quality manga reading experience. You don't have to worry about any illegal or unethical aspects when using it.
The main difference is probably in the style and storyline. Raw manga is often more unfiltered and might have a rougher edge. Kingdom manga, on the other hand, has its own unique plot and art style.