There's another option to upgrade unsupported Macs to Catalina, which will allow you to run TurboTax. I upgraded my 2010 MacBook Pro this way and it is running very solid on Catalina. Next will upgrade my 2009 Mac Pro to Catalina. I previously upgraded my MacBook Pro to Big Sur using the OpenCore Legacy Patcher but performance was slow. I'm sure the Mac Pro would have handled it better and that may still be an option.
In any case, this is a reputable upgrade and known issues are published. It will allow you to continue using TurboTax and kick the upgrade can another year or two down the road. You should note, however, for the Catalina upgrade, the developer is no longer updating things so any hiccups you encounter will likely not get resolved. I haven't experienced any hiccups in my upgrade, and there is a listing of known issues you can review before upgrading your OS. But this solution is working well for me so far. Good luck.
http://dosdude1.com/catalina/
