Creating Category Specific Menus in WordPress

First time I have used stack-overflow, please forgive me if I make a faux pas.

I am new to php, and am currently building my first WordPress Theme. I want to have several separate sections of the website with a side-bar menu that displays links only to the other pages in the section you are in. For example if you are viewing post 1 of section A the side menu would display links to posts 2, 3, and 4 of section A. The menu would be the same on all posts of section A. Section B would have a different menu displaying it’s posts. The site will require about 25-30 different sections.

I was hoping to do this with Categories, but am certainly open to other options. I have attempted this a few different ways to no success. I feel this should be simple, and I am just missing something basic.

I think my problem is calling the information necessary I started it as:

<?php if (is_category('tanzania')) : ?>

   <?php wp_nav_menu(array(
    'theme_location' => 'tanzania-menu'
   )) ?>

<?php elseif (is_category('safrica')) : ?>

   <?php wp_nav_menu(array(
    'theme_location' => 'safrica-menu'
   )) 

<?php else : print('Nope.'); ?>

<?php endif; ?>

This did not work at all (something I am sure is super obvious to you all). It displays each menu properly but only at the category overview page. Once you click on a link in the menu it takes you to the post’s perma-url and the menu vanishes. I figured out that this is because the in_category only sees pages in the /category/ directory, but am unsure how to fix it.

The other issue is having my sidebar.php be so long. Using the above method I basically had a huge switch ~80 lines long. I am not sure if that is bad, but it seems… unwise. Everytime I tried to shrink it down, I ended up with it not working at all.

Thank you for your help with this, WP has been quite the learning curve.