This OpenCart related products by Category module will allow you to quickly relate Categories to each other .
Get related products for OpenCart 3 - CLICK HERE
FEATURES

• Relate one or many to many products at once
  
• Filter products by price range
  
• Relate products by category
  
• Relate one or many product Categories to single or many product Categories
  
• Delete Related products for given Category of main products
  
• - one category to many categories Related products
  
• - many categories to one category Related products
  
• - one category to one category Related products
  
• - one product to whole category Related products
  
• - many to many or one products Related products

DEMO
(This is the full demo. In this product you will have only the Price Range and the Category tabs in Advanced view)
http://ggwshop.com/oc23bulkrelatedultimate/admin/
user: demo
pass: demo

Requires vQmod 2.3+
Installs via the Extension Installer
Old modules compatibility fix for OpenCart 2.3 included. So your older modules (for 2.1 for example) will magically start to work again.
Check the Documentation its a 1 minute read and install process I promise :)