I gained a ton of weight after I stopped smoking. To take it off and keep it off, I made several similar changes.

I stopped drinking sweetened drinks, cut out refined carbs, and took up learning to cook everything from scratch as my new hobby. (It's hard to really get the sugar in your diet under control unless you cook everything from scratch.)

I didn't change my level of physical activity.

This has worked to keep the weight off for more than a decade.