If you want to run CS 5 on your system you need to downgrade to El Capitan ("EC"). Here are the steps using the downgrade tutorial video.
- Have latest backup of your El Capitan system handy (on external drive)
- Follow downgrade tutorial video to get El Capitan installed on your system. Yes, doing this will temporarily erase everything on your system to the bare bone Mac OS. This is so you can restore your time machine backup and have it not default to Monterey.
- Restore your most recent El Capitan backup using time machine. This will put your machine back to just before you upgraded to Monterey. Back up using time machine.
This will get you back to your old system answering your initial request. If you'd like to upgrade your iMac to Monterey and still be able to use CS5, you'll need to follow this tutorial video: https://www.youtube.com/watch?v=_G-aVaj6GK0
This will clone your El Capitan system (with CS 5) onto an SSD external drive. After that has been complete (and tested), you can upgrade your system through the System Preferences > Software Updates. Then to run CS 5, simply restart your iMac, hold down the option key (before the Apple dong sounds) and you'll be able to select which system to boot from. Select the El Capitan external drive and your iMac will boot and you can run CS 5.
You'll also be able to access all your files from your internal drive if needed. To keep your El Capitan external drive full of space, you're free to delete apps, files and pretty much anything not CS 5 related or El Capitan core apps, etc. and have plenty of storage.
Outside of what others and I have provided here, you may want to hire a local Mac IT specialist and have them follow the steps provided on this thread. The $100-$200 will be well worth the effort if you don't feel comfortable doing this. :) Best of luck!