Mobile first - flex - first attempt ever at a design.

Did mobile without any issues, but really struggled to go from mobile over to Desktop, took me many many hours the image just wouldn't fit, and it messes up badly in the 1000px-1400px range, where have I messed up? I guess it fits the design for the actual 1440px and 375px spec but I plan to make all my designs full responsive on most devices. Am I doing something fundamentally wrong with how I'm doing my mobile so making it too hard moving over to Desktop? I didn't know where the hell I was with all the margins and paddings used haha.

Oh and that color on the image, the best I could get was with mix blend mode multiple with opacity's on the background and the ::before. Is there an easier way to match the design?

Any tips or advice would be mostly appreciated I'm mostly worried about bad practise and planning for mobile to desktop. Thank you.