Responsive Mobile-First Solution with Flexbox

This is my first Front-End Mentor project using Flexbox and a Mobile-First workflow. I am currently using a Mobile-First workflow on other Front-End Mentor projects under development, but I'm not always finding it easy. I understand Mobile-First, and I've read books about it, but I'm so used to creating the wide-screen version first that it feels as though I'm working backwards now. Sometimes the mobile version comes out fine, but the wide-screen version is a mess, and I have to change the HTML5 section markup to accommodate both versions. Frequently, it's not as simple as changing 'flex-direction' from 'column' to 'row.' Everything appears to work well on both my laptop and my smartphone. I would appreciate any suggestions on improving my code as well as on how I can better use a Mobile-First workflow. Thank you.