I agree with Ron, in this instance I really don't see any possible solution to updating your current iMac that would be worthy of the time, effort and expense.
You are right when you state that there has not been much in the way of new hardware since 2014 ... which means you can save a significant amount of money by picking up a slightly used or refurb model. Not really necessary to order brand spanking new. Just keep in mind, for the past few iterations, the 27" model is the only one that still offers user upgradable RAM slots ... with the 21", your are stuck with whatever was offered from the factory.