Old IMac 2012 not compatible with photoshop subscription

If you want to use a newer version of the Adobe software, then you will need a newer computer since Adobe now only supports the three most recent versions of macOS (currently Ventura, Sonoma, and Sequoia). Keep in mind the Adobe software is now all subscription based. If the old software still does what you need, then just keep using it. Or find some other software that may still work with your older Mac.

You have asked several questions. As with any application, your Photoshop must be operable with your installed OS. It is likely that Adobe no longer supports your older version. You need to address that issue with Adobe. Of course, a new computer will likely be able to run the current version of Photoshop. As to reusing your monitor with a new computer, most likely you can but that must be addressed with the monitor's manufacture. Check their web site for details on compatibility issues. If you purchase an external disk you can not install any operating system on it that the computer's HD will not accept. The limitation is in the computer.