+1 to @Niel's excellent guidance. 👍🏽
Because you have allowed your macOS to languish at macOS 10.11.6 El Capitan, a full six or seven generations behind the newest OS it could be running, upgrading it is going to be "painful".
Depending on its exact model, your 2015 iMac is capable of running up to either macOS11 Big Sur or macOS 12 Monterey. Major changes were made when macOS 10.15 Catalina was released in 2019. If you wish to upgrade to Catalina or newer, then you're going to have to evaluate the apps that you are using in El Capitan for 64-bit compatibility with Catalina, Big Sur and Monterey. As @Niel has suggested, use Go64 from St. Claire Software to discover which of your current apps are not 32-bit apps and will require updates or replacements.
If you want to get the longest life from this iMac, then you'll want to upgrade to either Big Sur or Monterey, whichever is compatible.
Had you kept your Mac up to date for the past 9 years, you'd have been using the newest OS three or four years ago and the transitional upgrades likely would have been easier.
Ten years is a good run for a Mac. If you upgrade to Big Sur or Monterey you still have browsers that can make secure connection to the internet. However, some big software packages from Microsoft, Adobe, Intuit Turbo Tax and others are no longer supported even in those macOS versions.
Were I in your position, I'd bite the bullet and upgrade the OS to Big Sur or Monterey and continue to use the Mac until the drives fall off, knowing that it is classified by Apple as an obsolete computer running a vintage OS.
Meanwhile, I'd be saving my nickels and making plans to replace the 2015 iMac.
