@stijn25802 Your posture actually doesn't look too bad. Possibly some slight anterior pelvic tilt, so you can check out the video below if you want to work on that a little bit. But things might change once you add a bit more mass anyway. The other thing is make sure you give your rear delts / traps plenty of love to avoid rounded shoulders.
FBE is definitely a good option, but you do need to be eating enough food to grow. What you're eating at the moment probably isn't enough and if you aren't gaining weight, that's a sure sign that you need to eat more.
As for food options, check this out! https://muscularstrength.com/article/healthy-shopping-list-for-life