If your 21.5 inch iMac has an Apple rotational drive, then it will be slow no matter what operating system you install on it.
I recommend installing High Sierra 10.13.6, since it is still covered under Apple's operating system security and Safari browser updates until after macOS 11.0 (Big Sur) is released this Fall. High Sierra will be the last chance to get the current Safari 13.1.2. Also, presently, if you wanted to re/install MS Office, Microsoft has deactivated the Office 2008/2011 activation servers for any operating system installation, and does require at minimum, High Sierra 10.13.6 for installation of Microsoft 365, or Office 2019 for Mac.
View the following link with Safari only: How to upgrade to macOS High Sierra. There is a link there to get the High Sierra full installer. Always a good idea to boot into Recovery (⌘R) and run Disk Utility First Aid on the internal drive, and after a normal reboot, have a last Time Machine backup of the current operating system before performing any upgrade.
Now about Pages. High Sierra requires Pages v7.3, as v5.6.2 is too old for it. Although Apple does not provide manual downloads of previous Pages installers, the Mac App Store may help you out. Once High Sierra is installed and updated via the Updates panel in the Mac App Store, you can press the option key and the Purchases tab in the Mac App Store, and this should show you a Pages icon with an install/update button. Click that. It will inform you that it cannot obtain the current version of Pages in the Mac App Store, but may offer to install the latest version for your operating system. That would be Pages v8.1.
Pages v8.1 opened Pages '08, '09, and v5.1 - v7.3 Pages documents without an issue for me. If someone sends you a Pages document from Pages v8.2 thru v10.1 (currently), and they do not use features unique to those versions, then you should be able to open those documents with Pages v8.1, and even v5.6.2 if you remain on El Capitan.