I love flex, it's made CSS so easy. One of the things that improved my flex usage was using Penpot to draw my designs before implementing them.
They have alignment properties for graphical elements that work like flex's justify-content and align-items properties so once you design a view in Penpot it becomes almost trivial to translate it into HTML/CSS using flex.
It really changed my mindset from working with relative or absolute positioning, blocks, margins, padding, etc... to simply working with flex everywhere. And it's responsive automatically!
They have alignment properties for graphical elements that work like flex's justify-content and align-items properties so once you design a view in Penpot it becomes almost trivial to translate it into HTML/CSS using flex.
It really changed my mindset from working with relative or absolute positioning, blocks, margins, padding, etc... to simply working with flex everywhere. And it's responsive automatically!