The anime of My Dress-Up Darling typically ends at a certain point in the manga, but it's not always an exact match. Sometimes, adaptations make slight changes.
I'd recommend starting from the chapter where the anime ended. That way, you can seamlessly continue the story and discover new plot developments. But if you want a full experience, starting from the first chapter isn't a bad idea either.
Not at all. My Dress Up Darling manga is far from being cancelled. It continues to attract a large fan base and the creators are actively working on new chapters.