Apple wants people to replace Mac about every 7 years, when they usually declare them vintaged. You've squeezed about 10 out of yours. This is the Apple way. They want more money from their customers ASAP so they can pay their own turbo taxes and enjoy the net, faster and faster. I suspect they'd love to get Macs on the iPhone replacement schedule if they could pull that off.
As others have offered, your best option is use bootcamp to boot into Windows which should easily let you use TurboTax. Windows tends to support hardware much longer than Apple, so you might be good for another few years minimum. If I'm not mistaken, there's now a way to install the latest Windows 11 on that iMac. If so, you would basically be up to date with latest Windows.
However, if you have to have a Mac, Apple wants another big donation from you. And then start saving to do it again in about 2031 or so... UNLESS, the do- in fact- figure out a way to accelerate the upgrade cycle... perhaps with iPhone-like "long in tooth" mysterious slowdowns with each macOS update.
Else, you might consider trying the OCLP hack which- among other things- proves that vintage hardware can generally run updated macOS just fine. IMO, the main risk here is trusting what is basically a third party hack to not introduce brand new kinds of security risks not necessarily prevented by an updated macOS.
Name any appliance that does more year after year than what it did when released. His Mac does exactly what it was designed to do. It’s not greedy Apple, it’s progress. Don’t like it? Stick with what you bought.