The difficult part was positioning the hero mockup image and the background Image Also.
I recommend using the mobile-first approach almost always, not because it is right, but because it is more convenient. From the client's perspective, for instance, many people will likely access your webpage via mobile devices.
As a designer, I think it is easier to implement the small screen version, and then adapt your design for a larger screen, since in most cases, very few elements/components of your design actually change as you advance from a smaller screen to a larger screen and vice versa.
I also ran to the challenge with paddings/margins when using a "desktop-first" approach. the values for larger screen sizes are sometimes set to bigger values, and they do not scale down with the screen sizes. this essentially affects the layout of the mobile and can be overwhelming for a beginner.
