Looking back would it have been easier to do the whole things in JS/HTML/CSS?

Not at all! This is a mobile game and getting 60 FPS on mobile in a browser is not trivial, even with graphics as simple as these. But natively? Ridiculously easy.

The game is simple/hard and beautiful.

I am just curious. What engine and development tools did you use? If I like a game, I want to know the backend tools that was used for developing.

That'd be great :)