Good news, I figured it out!
I had to copy the loop.php into a new file called loop-category.php, and in this file, change <?php the_excerpt(); ?> for <?php the_content(); ?> and voilá! Category pages are now blog-like, and other pages still show excerpts!
Oh, but after doing this it still generated the excerpt thumbnail, which messed up the posts. So, I also had to remove the code right above that line to remove the thumbnail generation, and now the posts are a-okay. I’m very happy now! Thanks for your help though.
(I can post a more specific step-by-step guide to help others, if you think that’s a good idea. 🙂