Downgrading to El Capitan should work using the same steps Here's the install file for El Capitan: http://updates-http.cdn-apple.com/2019/cert/061-41424-20191024-218af9ec-cf50-4516-9011-228c78eda3d2/InstallMacOSX.dmg. Came from here: https://support.apple.com/en-us/HT211683 (scroll down for Sierra, High Sierra, etc).
Once you're able to downgrade to EC and restore your system from your time machine backup, you can try upgrading to High Sierra (or not) through the Software Update in System Preferences. Then test your Adobe software. If all is good, you can then follow the instructions I shared to copy your system to a bootable SSD. From there test booting from the SSD (hold down the option key when powering up) and then try running your Adobe software. If your CS 5 suite works, then you can safely upgrade your main iMac OS to Monterey.
I must admit, my old MacBook Pro 2015 Retina runs a lot faster using Monterey and some of the UI features and integrations with our other Mac products (iPhones, Airpods, iPads, Apple TV, etc) are seamless and snappy. Good luck!
[This is a Direct Download Link]