Your Late 2012 iMac can install up to Catalina, I sure would not use El Crapitain. What exact model/EMC code is it? esp. 21.5" or 27" How much memory? 16GB which is the max config is really the minimum I want in a modern Pro Tools system.
My choice would be Mojave or Catalina. They you can run the latest available web browsers, latest available version for Pro Tools, etc.
Be aware of Mojave cautions.
https://avid.secure.force.com/pkb/ar...5-Release-Info
Pay attention to those DRAM requirements. And I hope you are not running on an internal slow HDD... it may be time to crack open or have have somebody upgrade that iMac for you if needed.
Do not shoot yourself in the head here, get a spare external SSD drive or partition and do a full clean install of macOS on that, installing clean copies of Pro Tools, ILM, plugins and other apps, testing as you go, then migrate the user account using the system migration tool (do not migrate apps or drivers). Then once all that is done and running OK clone that disk over the top your current boot partition. Current versions of macOS will utilize APFS filesystems, be aware of that and any impact in sharing media between other systems.