HTML and CSS solution using lots of Flexbox

First of all, thanks for looking at my code ^^. I had some problems figuring out, how to change both the mobile and desktop design without interfering with each other.I tried to use one media query, but it somehow didnt work. Then i came to the solution to use two media querys. Therefore my css code got way too long(?). Is it practical to do it this way or is there an better alternative. I also want to know, if used "display: flex" too often? Is it bad to do it and why? If there is anything that is worth mentioning, pls do so. I appreciate every comment !