I would advise you to upgrade to 10.14 Mojave. It's the last upgrade before Catalina that won't be limited by all your 32-bit apps.
Your iMac should run Mojave just fine.
How to Upgrade to macOS 10.14 Mojave:
❗️– Backup your data. (You’ve been warned.)
Learn How to Back up your Mac with Time Machine:
click > https://support.apple.com/en-us/HT201250
📚 – Check Compatibility of your Mac with newer versions of macOS.
Learn how to upgrade macOS:
click > https://support.apple.com/en-us/HT211683
These Mac models are compatible with macOS Mojave:
• MacBook (Early 2015 or newer)
• MacBook Air (Mid 2012 or newer)
• MacBook Pro (Mid 2012 or newer)
• Mac mini (Late 2012 or newer)
• iMac (Late 2012 or newer)
• iMac Pro (2017)
• Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with a
recommended Metal-capable graphics cards.
For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac.
✅ - Download the macOS 10.14 Mojave upgrade installer here:
https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12
(Don’t use FireFox for this link)
The download will place the "Install macOS Mojave" application in the Applications folder. Double-click the installer to begin the upgrade process.