Sunnyside agency landing page with flexbox and js vanilla

I have some questions, can anyone help me?

is it ok to use flexbox to simulate grids?

I have a box with two others inside, divided along the line, but one of the inside boxes is growing more than the other, I don't know how to solve it without breaking the flexibility of the other break points.

Also, I'm underlining "learn more" with a :after, is there any way to make it stick with the sentence without having to change the placement at each breakpoint?