Ok - let's get a few things straight.
1. A 2009 24" 2.66 GHz iMac cannot be updated past 10.11 (El Capitan). See:
https://everymac.com/systems/apple/imac/specs/imac-core-2-duo-2.66-24-inch-aluminum-early-2009-specs.html2. Raven is currently using
10.10.5 - not 10.5, guys.
3. For future reference for anyone else considering updates, Adobe CS6 will work fine in 10.13. I am still using everything no problem, other than InDesign looks like absolute crap, as does Acrobat. (not optimized for Retina screens, is my assumption). I am, however planning on transitioning away from Adobe now that they've broken their promise to keep the non-subscription option for Lightroom. There are lots of up and coming alternatives out there now - some of them very promising.
4. Never had any issues with fonts through all my updates. (Don't have quite as many as you have - but do have close to 500)
5. Your peripherals should be fine with the update to 10.11, but should you decide it's time for a new Mac, you should check on the manufacturer's website. Epson does say they will provide support for Epson products for 10.14, so you should be good to go. You'll need to check Lexmark's site.