Julia Lovell's eponymous book on the Opium War is a good one to approach the subject. It's reasonable length, covers the whole topic, and has a nice epilogue on the importance of the story in current Chinese politics. She also did a thing for Fivebooks.com about the five best books on the Opium Wars. https://fivebooks.com/best-books/opium-war-julia-lovell/

Stephen Platt's book on the Taiping Civil War, Autumn in the Heavenly Kingdom, is interesting. It's mostly about the US and British response so it would fit in well with your interest in the colonial aspects.

I read a book a few months ago by Helen Zhi called The Last Boat Out of Shanghai about the diaspora community in Shanghai and it had info about the colonization of Shanghai by the western powers and the Japanese. It's a good book but only about half of it deals with the topic your interested in. James Carter has a book called Champion's Day about the Japanese occupation. And Paul French has a kind of a lurid but fun one called City of Devils about the underworld in Shanghai.